Yegg was presumably abandoned under a tree as a bappy (they might have ran away). Nothing to worry about, as Perp and Newt quickly found her and decided they would adopt her. Why not?

Yeggy grew up in a chaotic environment; Newt and Perp were on the run at all times. The Perpetrator breaks symp laws constantly, living the arsonist's dream- burning down nearly every house in sight. (Only after robbing each household blind! Hey- gotta make a living somehow! Why not have fun with it?)

Yeggy, now an adult, follows a somewhat similar path. She does steal for a living, but isn't so quick to get caught. Charming and disarming by day, sneaky and thiefy by night- they love the life they live. Yegg's partner, Horizon, follows along happily. (He likes precious jewlery, and loves stealing it even more. They're a great match of harmonious contradictions)

Yegg suspects that they may be lactose intolerant.