It may contain SPOILERS.
Continue reading at your own risk.
Viper is an Arcobaleno that holds the Mist Arcobaleno Pacifier. He is a member of the elite Vongola assassination team, Varia, under his chosen name Esper Mammon. Known as the Varia's best spell caster, Viper has the ability to cast illusions as well as being able to locate any person by sneezing into a piece of paper.
Both as an adult and in Arcobaleno form, Viper is shown wearing a black outfit with a hood that covers his eyes, making it hard to know his gender. Before becoming an Arcobaleno, Viper is seen to have chin-length purple-colored hair and fairly white skin. He has two triangular purple markings on his cheek that extend to his jawline when he use their Arcobaleno powers. In his Arcobaleno form, his mouth is shaped like an upside-down V.
He also have a dark green frog named Fantasma that lies on top of his head. His Arcobaleno Pacifier is covered with the Mammon Chain, preventing other Arcobaleno to sense him while also preventing the use of his powers. The chain is replaced by the Vongola by the cap that conceals the Rings' powers the same way as the Chains.
As an Arcobaleno, they carry with him a roll of what seems to be tissue and uses it to create illusions to attack his opponents.
As the representative of Greed, Viper only works when money is involved, with the exception of the Arcobaleno Trials. He is materialistic, and is generally apathetic, though he showed more emotion when they were trying to break the Arcobaleno curse, and vehemently expressed his desire to continue living. He tends to charge very high prices for his services.
Viper is one of the world's "I Prescelti Sette" (Strongest Seven) and was thus selected to become one of the Arcobaleno. At some point after he became an Arcobaleno, he joined the Varia and participated in the Cradle Affair. He then remained in the Varia for the following eight years until Xanxus was freed.
The Varia's plan in the beginning was to find all of the Vongola half-ring bearers on Tsuna's side and take the half-rings by force. Viper used his Thoughtography to help the Varia members locate its equivalent member on Tsuna's side. Before a fight happened, CEDEF interrupted and it was explained that there would be an official battle for the rings. Members on both side will fight for the other half of their ring against the member on the opposite team who has it. Being bearer of half of the Vongola Mist Ring, Viper fought against the corresponding Mist guardian, Chrome Dokuro. He revealed himself as the Mist Arcobaleno and easily bested Chrome. When it seemed over, Chrome switched bodies with Mukuro Rokudo. Viper at first didn't believe it was Mukuro because he had heard of his imprisonment at Vendice, but he soon came to realize it was. Being the Mist Arcobaleno, the strongest illusionist, he was frustrated when Mukuro was overpowering him. He asked where Mukuro got this talented and Mukuro simply responded that he got his powers from Hell itself, irritating Viper, who didn't believe him at all, believing that Mukuro's statement was merely rubbish. Viper was defeated and seemingly exploded, but Mukuro revealed that Viper had survived because he had reserved enough energy to escape as a precaution. As things progressed and events happened, Varia members are shown to have caught Viper from wherever he was regenerating to help fight at the main battle of the Ring Conflict, where whichever boss gathers all the rings first will succeed as the 10th Vongola Boss. As Xanxus and Tsuna fought, all the guardians were poisoned with a paralyzing illness that only obtaining the ring of their type would give them the antidote. Varia member Belphegor came and healed Viper and they set up a plan for getting all the rings by holding Chrome hostage. Their plan succeeded and even though their boss had lost against Tsuna, they gave him all the rings to seemingly take the win. The Vongola Rings rejected Xanxus and it became known that he didn't actually have Vongola blood. The Varia was disqualified but Belphegor revealed that backup was coming, backup that was only bested by the main Varia members. Three people arrived, nearly unconscious, and reported that all the rest had been defeated by Lancia. Being the only two members of Varia left able to fight, and being surrounded by all 6 of Tsuna's guardians, Belphegor and Viper surrendered.
Viper is the third administrator of the Arcobaleno Trials. He and Belphegor enrolled in Tsuna's school for a time, using the pseudonyms Monta and Himeko Hariyama. Later, Takeshi Yamamoto discovers this when Himeko throws some of Bel's signature Knives to protect himself and Kyoko. Viper's Trial involves battling Tsuna, Ryohei Sasagawa, and Yamamoto. In the Trial, he use his special Ability: Mammon Mirage. Everyone was surprised when they heard Verde told Viper to test out a prototype Box Weapon. Reborn said that he had a bad feeling about this. During Verde's trial, Viper attempts to stop a Pawn missile from making it to Namimori, although his efforts was thwarted by a substance which was later revealed to be similar to the Non Tri-Ni-Sette, which Verde uses to capture the rest of the Arcobaleno, sans Aria. Unlike the actual Non Tri-Ni-Sette, the substance only paralyzes them, as opposed to outright killing them. It later turns out that the captured Arcobaleno were revealed to have been Illusions done by Viper as soon as he saw the substance released again when all the Arcobaleno were present.
Viper and the other Arcobaleno are called by Reborn to train Tsuna and the others. Viper is paired up with Chrome as his student. It seems however, that Viper hasn't taught anything to Chrome due to the fact that he wouldn't get paid. So, right after he comes back, after hearing their insults from Skull, he leaves. However, he come back when Colonnello is searching for him and later helps Chrome to break Daemon Spade's Illusion. After Daemon and Giotto gave Chrome and Tsuna their inheritance, Chrome thanked him for saving her, and he said that she had improved, somehow. Shortly, he was picked up by Squalo and Belphegor who had searched for him everywhere because there was a job that required an illusionist.
At the ceremony, he noticed that Dino and Superbi Squalo had seen through Chrome's illusion of Yamamoto; however, he tells her that an illusion of that level can be only seen through by experts and assures her that she was doing well. At the last minute right after Tsuna and the others were defeated by the Shimon, he and the rest of the Varia along with Dino and Reborn, tried to stop the Simon but was stopped in their tracks by Adelheid. Squalo requested for them and the rest of the Varia to chase after the Simon right after they took Chrome away, but the Ninth order them not to chase after them.
Reborn sent a message to Viper and the other Arcobaleno about a hint about their curse. Viper was about to read the rest of the letter until Belphegor threw food at Viper which ruined the letter. He at first was confused of how to read the rest of the letter but then when Belphegor says that they will go to Jura in France to recruit Flan, Viper remembers the Pacifier Spring that was located near the area which could strengthen the light of pacifier, which could perhaps allow him to read the rest of the letter, and decides to go there along with the other Varia except for Xanxus. Viper and the others found Mukuro and the Kokuyo Gang also trying to recruit the young Flan, but Viper leaves soon afterwards, saying he couldn't care less about Flan. Viper heads to Pacifier spring and finds the other Arcobaleno, Fon, Colonello, Skull, and Verde. They said that some of the letters were unreadable, which can explain why some of the unreadable words on Viper's letter were not caused by the food. They say they should stack up the letters for the missing words to fit. After that, they know the information Reborn got during the battle against Daemon Spade, including the one about the mysterious Arcobaleno from the Vindice.
Viper, along with the other Arcobaleno, minus Aria, are in a dream they are all having. They meet the mysterious man that they met before they became the Arcobaleno. The man asks if they want to be freed from the curse of the rainbow. Viper shows himself to be very passionate about breaking the curse, saying that it's been his dream for many years. The man says that the Arcobaleno that gives the best contributions will be released from the curse; he then reveals that the one that shows himself to be the strongest will be considered worthy. He states that the Arcobaleno cannot personally fight for themselves, but must get representatives to fight for them. Viper is then shown back at Varia headquarters requesting assistance from Xanxus, who replies that it depended on the situation, but is later on revealed to have accepted. He is later seen by Dino in Japan with the rest of the Varia, who were staying in the same hotel as the Chiavarone Famiglia. Viper is later seen in their residing hotel riding on Belphegor's head as they kidnapped Tsuna, calling Tsuna a brat; a while later, after Wonomichi arrives in the Varia's hotel room, Viper connects the former's appearance and clothing to Checker Face's. He then listen to the rules of the Representative Battle of the Rainbow.
Later, on the second day of the Representative Battle of the Rainbow, Viper tries to get Xanxus to fight Team Fon when they came to challenge them. Viper is successful and watches the battle, but is forced to release his curse with their Rainbow Wristwatch and use Hyper Mirage R to stop Fon from eliminating Xanxus and eliminating them from the battle, and announces that the rule they set for the illusion was that the side that doubted their victory would self-destruct. He sent Fon and himself into illusionary space that he created and started to fight him one-on-one. However, Fon easily destroyed his illusions, making him doubt winning. Fortunately for him, Fon's curse release time has reached its limit and he returned to his baby form. Seeing this chance, Viper freeze Hibari's legs to prevent him from escaping Xanxus' attack, but Dino saved Hibari before the attack hit him. Xanxus ordered Viper to stand back since he's injured. Viper at first protested, but then reluctantly obeyed when Belphegor said that Xanxus wants to fight alone.
After the second battle ended with a draw, they and the other Arcobaleno are shocked to see Bermuda along with seven Vindices joining the Representative Battle of the Rainbow as the eighth team. Shortly after, Viper and the other Arcobaleno visit Skull at Namimori Hospital and discuss Bermuda, but find nothing on him. He then question Reborn his reason to join the Representative Battle of the Rainbow, but Reborn refuses to answer and instead tells them to ask Fon and Yuni. After the meeting dismisses, he go back to the hotel and are surprised to find that his team has been assaulted by a Vindice.
On the third day of battle, Team Mammon is attacked by three Vindice. Viper goes into their un-cursed form and uses their Illusions to help their team escape the Vindice. He later meet with Reborn and the other Arcobaleno and opposes Reborn's idea of teaming up with Bermuda. This leads to a fight in which Viper accidentally hits Yuni with an illusionary attack.
He is last seen with the other Varia members when Tsuna thinks about the people he (Tsuna) can rely on.
Mammon joined the Varia after being cursed. Notably, he changed his name and his fellow Arcobaleno weren't even sure if he was alive, showing that he might prefer the Varia to the Arcobaleno. Belphegor is the Varia member he spends the most time with. In the anime, the two even briefly attended school together. When fighting against the Vindice, Xanxus impulsively tried to destroy his wristwatch, which would have resulted in Mammon's elimination, the other Varia captains immediately pounced on Xanxus and prevented him from doing so. Lussuria even said that Xanxus shouldn't go against Mammon's wishes.
Of all the Varia members, Mammon is the only one who hasn't directly interacted with Flan so far. Flan was forced (by Belphegor) to wear a frog-shaped hat as a stark reminder of being Mammon's replacement.
Chrome is Mammon's student. They competed for the Mist Vongola Ring and Mammon was perfectly ready and willing to harm Chrome. Mammon also cooperated with Belphegor to tie up Chrome and threaten to kill her, though fortunately for her, Gokudera and Yamamoto saved her in time. Mukuro later intervened on her behalf. After the Varia's defeat, Mammon was paired with Chrome as her Arcobaleno tutor. Colonello warned him not to hold the outcome of their Ring Battle against her, to which he calmly replied that he doesn't. Mammon then rescued Chrome when Daemon used her and the other girls as bait.
Mammon dislikes Fon, though Fon may not necessarily feel the same way towards him. He also resents Reborn for not resisting harder against their tormentor Kawahira. His relationships with the other Arcobaleno are not shown in detail.
Weapons and Abilities
Before he was turned into an Arcobaleno, he was a first-class psychic who added Illusions to his repertoire. It was seen he could also fly before he was turned into an Arcobaleno. His psychic ability was telepathy, and he could also dowse things through pieces of paper so that he could find objects and people. As an Arcobaleno he still has all of his powers, but his Illusions have been strengthened, possibly because the Arcobaleno Mist Pacifer is an object of the Tri-ni-set that emits Mist Flames, creating real Illusions, and giving the wielder the Arcobaleno's ability to emit Dying Will Flames from their bodies.
- Illusions: As the Mist Arcobaleno, his Illusions is first class; the only time he was defeated by another Illusionist was when he fought Mukuro during the Mist Ring Battle. His most commonly seen used Illusions are Illusions of clones of themselves and the Illusion of tentacles.
- Viper Mirage: One of Viper's many Illusion abilities; he used this to surround Namimori with a series of mirages.
- Viper Mirage R: One of Viper's illusions that sets a rule on the target of which the illusion is cast on. If the rule is broken by the target, the target is instantly damaged with a multitude of deep cuts.
- Thoughtography: The ability to locate any person by sneezing into a piece of paper; the spit or mucus that sticks to the paper shows a map leading to the desired person.
- Flight: Viper can fly because of his Animal Partner Fantasma's abilities, but he has been shown to be able to fly without Fantasma's assistance.
- Uroboros: Declared by Viper himself to be his strongest illusion, Viper unleashes multiple large blind eel-like creatures with large mouths and sharp teeth.
- Mist Starfish: It was one of the first Box Weapons that were created and appeared in the past. It was used by Viper against Yamamoto and Ryohei during the 3rd Trial. When used, many spinning Starfish that emit Mist-Flames come out of it. Those that break are fused together and create a single and bigger Starfish.
- The word "Mammon" is a term used to describe material wealth or greed, most often personified as a deity.
- He dislikes being called by his given name, Viper.
- The name Viper refers to a type of snake, reflecting his choice of animal partner.
- His favorite word is 'greed', reflecting his own name.
- His favorite food is 5-yen chocolate, while his favorite drink is strawberry milk.
- His hobby is checking his deposit amount.
- He dislikes leaving his debts unpaid.
- Viper's gender has been a debate among fans due to the use of gender-neutral pronouns used to describe them. He was grouped into the male character category in the third popularity poll, and his gender continues to remain as male in the official VIZ translations, though VIZ itself is not "official" and has become infamous for making many errors and not adhering to the original text when translating.
- Viper has a character song: Maboroshi no Arcobaleno.
- In the official English translation, Reborn and Colonnello said that his Arcobaleno Pacifier had a "sunny glow."
- In Drama CD: Namimori Radio, it was revealed that he and Belphegor once borrowed one of Leviathan's parabolas when the weather was rainy, but ended up being struck by lightning.
- In Haru Haru's Interview: Dangerous, it's revealed that like Fon, he cannot eat Mapo Tofu since he turned into a baby because it was spicy, which makes him choke and tear up.
- Viper only likes to work if he gets paid. On rare occasions, Viper works for free only if the situation is dire or out of his duties as an Arcobaleno.
- Viper and Belphegor appear to be very close, as Viper avoids most of the Varia members and is mainly seen with Belphegor.