Sabine Callas hails from the United States, with possible links to Seattle. Holding a doctorate and awards such as the R. Francis Prize and the Denton Outstanding Innovation Award, her previous occupations include roles with Kingdom Corporation, eventually becoming its first Chief Scientific Officer on the board of advisors, and influence over the conglomerate's ventures in Rabat, Morocco. At some point during her time there however, there was an "incident". Little is known about what happened, but everything changed for Callas at that point after all she lost because of it, leading her to isolate and dedicate herself to pursuing vengeance instead.

After the events of First Light, the secretive VALORANT Protocol was founded, with Callas being one of its founders and becoming its second agent, "Viper", and second-in-command to Brimstone. One of its most active members as well as most experienced, Viper is involved with the recruitment of new agents and is active in many of their missions.

Viper is a savage and confident agent who doesn't fear enemies, and in fact desires the enemies' fear. She does not hesitate to call herself a villain and a monster, and will act like both if it means the success of her objective. Just like her abilities, she can be quite "toxic", however she is seen being more amiable and caring towards her teammates, but she will not hesitate to reprimand them harshly should they make major miscalculations in the battlefield.

Several agents have observed that Viper bottles a lot of violent anger within her. Viper has implied that her enemies have taken many things from her, and that she will stop at nothing to achieve her revenge.

Attracting attention with her jet black hair and venom green eyes, Viper also has a black mask. When Viper uses her ultimate ability, this mask turns into a gas mask which covers her whole head. She dons a green skin-tight body suit accompanied by sharp black shoulder plates, and wears customized gloves wherein she can release her toxic gasses. She wears black boots that reaches her thighs, and she carries a container on her back which carries her toxins.