Viper joined the Varia after being cursed. They changed their name to "Esper Mammon" and their fellow Arcobaleno are unsure if they were alive, showing that Viper might prefer the Varia to the Arcobaleno. Belphegor is the Varia member they spend 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 Viper'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 Viper's wishes.
Of all the Varia members, Viper is the only one who hasn't directly interacted with Fran so far. Fran was forced (by Belphegor) to wear a frog-shaped hat as a stark reminder of being Viper's replacement.
Chrome Dokuro is Viper's student. They competed for the Mist Vongola Ring and Viper was perfectly ready and willing to harm Chrome. Viper also cooperated with Belphegor to tie up Chrome and threaten to kill her, though fortunately for her, Hayato Gokudera and Takeshi Yamamoto saved her in time. Mukuro Rokudo later intervened on her behalf. After the Varia's defeat, Viper was paired with Chrome as her Arcobaleno tutor. Colonnello warned them not to hold the outcome of their Ring Battle against her, to which Viper calmly replied that they don't. Viper then rescued Chrome when Demon Spade used her and the other girls as bait.
Viper dislikes Fon, though Fon may not necessarily feel the same way towards them. They also resent Reborn for not resisting harder against their tormentor Kawahira. Their relationship with the other Arcobaleno are not shown in detail.