He was originally posessing Somers' sister as part of a plan to influence him and eventually possess him, and now lives with Somers, Varg, Moya, and Jill, continually partying.
It's been stated by Cate (somewhere on tumblr, possibly in the commentary in lamezine 002?) that details like who the kid was before Ru'mel showed up, and 'ru's past form are details she's not gonna go into.
"Ru'mel: grown man demon in young girl body trap. never sober. GOURMET/INTERIOR DESIGN. respects CLASS [DESIRES MOUTH GRILL AND VARIOUS DIAMONDS/WOULD READILY DRESS LIKE ALIEN FROM SPRING BREAKERS]"[]