Thing is, Bioware doesn't use labels and feels no need to point out who wants to stick their what in who. Much as the world should be really. Hence there are already Bioware characters that arguably fit into this terminology.
Um, what? 
Okay, I'll just stick with the DA games here. You are very, very wrong. While the characters in these games may not use the terms, the DA team does and preference comes up quite a bit.
Origins:
Oghren talks about sex darn near all the time.
Leliana mentions her female lover, Marjoline.
Wynne talked about having a child with a male templar.
Zevran is open about having had sex with both men and women.
See: Every bit of party banter about your LI choice/status.
DA2:
Depending upon gender, Anders does mention he had a male lover. In both cases, he mentions having sex with others in the Circle.
Isabela talks very bluntly about having sex with both men and women.
Sebastian is open about having a lot of sex with women in his past.
Fenris and Isabela are a couple if neither are romanced by the player, and talk about sex.
See: Every bit of party banter about your LI choice/status
DAI:
Sera makes it very clear that she finds female Qunari attractive to matter what.
Dorian's personal quest. Full stop.
Iron Bull talks about sex all the time. To the point where NPCs in Haven are talking about it.
Iron Bull and Dorian are a couple if neither are romanced and will openly flirt in party banter.
Josie and Blackwall have a romance if neither are with the player.
See: Every bit of party banter about your LI choice/status.
BioWare uses labels all the time for describing their characters as well as having straight and non-straight characters talk about their sexual history/interests/preferences. And as far as any character that would have fit as demisexual, that would have been Merrill. That the DA team said was bisexual early this summer. That is a specific label and term that BioWare used. By that term alone, she (unfortunately for the OP,) is not demisexual. So far, while there have been a few you could squint and call demisexual, there hasn't been a companion that fits that definition yet.
Also, at GamyerX this year they talked about writing an asexual romance as well. So I think that Hana's idea could tie into that.