This is in fanfiction. Ever since Tekken Tag Tournament 2 when all of the guys can wear either a swimsuit or a fundoshi with a rear apron. The first time I've seen Jin or his devil form looking like that, he has no butt floss like the other guys. So this is why he was an easy target on pranking. Personally, Jin was extremely sensitive if he wears more revealing outfits to show more of his bare skin (He was already succeeded by Ryu because he too also wears Jin Saotome's fundoshi without a rear apron as one of his alternate costumes in SFV). Ling Xiaoyu had already do that to him once by accidentally goosing his butt and was excited because she states it was soft and fluffy (and why she's more crazy over him).
