Sensei had a husband who died or she is a divorcee. Maybe she has a son? Maybe she's married and unhappy. That's why there's a picture facing the wall? Speculations are endless.
Well, Akari and Daichi awakened together the lingerie fetish, so Akari it is not really doomed to be completely unnoticed by Daichi. Also, that picture does look suspicious, something about her past, but what is a mystery.