Viper is the female member of the Regulars in Bakugan Battle Brawlers: Battle Trainer. She is an Aquos battler. She battles Dan after he defeats Aurock three times. She refers to Dan as her "favorite earthling".
Viper first appears after Dan has beaten Aurock three times. When Dan challenges Aurock, he instead finds Viper, who comments that he was the one who made Aurock "miserable." She then notes that Dan is a bit cute, and asks if he wants to be friends with her. When Drago tells him that the Regulars (whom he calls Guardians) were not their friends, she says, "Ignore him, Daniel. He's just jealous!" However, Dan manages to get back on track and challenges Viper. After he defeats her, she tells him that she'll be much more difficult to defeat the next time they meet.