User blog comment:Brandonmatias/Yami's "dis-likement" to Ecchi things./@comment-71.182.198.201-20140927035815

Here's a theory that I think is very much in keeping with the spirit of the story: Because she actually really enjoys it, and therefore it would be a distraction in battle, e.g. imagine Momo trying to fight someone who keeps fondling her tail. So, just like Yami not sleeping deeply, she's trained herself to instinctively dislike anything that turns her on.

By this theory, in "Darkness" mode she becomes cruel and destructive, but otherwise the apparent difference in personality is only due to her inhibitions being removed, letting her act on desires without regard for others. (yeah, she totally has a tentacle fetish, just sayin')

...and of course this would also explain why she reacts so strongly to Rito's perpetual mishaps, while with other people she generally does the minimum needed to make them stop. 'cuz everything is way more of a turn-on when it's Rito, heh heh.