All of these are yaoi's btw
Enzai, hands down, although I bout you could really show it on here, 'cause it's really really uuh, bad? But the main chacter gets thrown into prision, beat up, and sextually abused... a lot.
Another one that might fit if you go with a Yuki/Eiri pairing would be gravitation... It's a softish yaoi and
a few softer ones would be something like Yami no Matsuei (Descendants of Darkness) Hisoka would work there a bit if you put him with Maraki, Sukisho! they both kinda get abused as kids, No Money! (Okane ga Nai) the main chacter gets sold into slavery or something. Kaze to Ki no Uta (Song of Wind and Trees) that ones going out on a limb a little bit, the main chacters pretty much a whore and his uncle "trained" him to be like that but it doesn't show to much