wait who? There's not many "demon guys/incubus" period, as most prefer to be female. Only one I can think of is the guy with "Amber" and the two maids. I don't remember ever seeing him mentioning being in love with lyssieth though, just fawning/respect for a daughter lilin.
Did he give you "Steal" and if so how? I've never seen it legitimately rewarded in game, only through "buggy".
Only guy I know of who kinda mentions being "in love with lyssieth" is Arthur, but he isn't a demon, just a pure filthy degenerate human. He gives other major forbidden spells for quests, but never seen him give steal.
Tl,dr. True if big, need details.