Einlion
Ordinarily I'd be all for adding Setsuna, and a couple other 1-B Girls are good too, but yea, for this story, keeping it to 1-A and maybe a couple others is for probably for the best. Maybe if the Harem was pre-planned and/or wasn't all of 1-A from the beginning she'd have been a good candidate. Honestly, it's fine what it is now, 1-A, Mirko, Kinda Mei, Kinda-sorta/maybe Midnight. Maybe in the future Melissa Shield. I really hope it doesn't go Inko, it'd be disappointing to see the character and you as an author cave to it, but if it does, it does, and the commenters really seem to want it for some reason.