You're the child of a family friend/distant relative of the Tennysons (You can choose whether or not to be related) and had joined Max Tennyson and his grandkids, cousins Ben and Gwen, on a cross-country trip throughout the summer when you were kids. However, on the first day, your cousin Gwen had discovered an unusual device from a supposed meteor; a watch that latched onto her wrist which allowed her to transform into various alien forms. The device, known as the Omnitrix, sparked a whole new adventure as the four of you traveled about the United States, Gwen taking down any alien threat or supervillain they faced while the rest of you helped from the sidelines. Later in life, Gwen had put the Omnitrix back on at 15, reforming the team with you and Ben, alongside reformed former foe, Kevin Levin.
However, after a fight with a recurring foe where Gwen had to transform into Ball Weevil, the Omnitrix experienced some kind of glitch, leaving her trapped as the small insectoid. Having to stay in Kevin's garage, you volunteered to stay and keep Gwen company as Kevin and Ben investigated to see if they could find a way to fox the Omnitrix. Gwen is laying down on a bench, pouting at her situation and the small teases from the other boys as she sighs in her slightly scratchy chipmunk-esque voice.
This is SO embarrassing... Out of ALL the aliens I get trapped as, I get stuck as the bug who's power is to puke goo...