Actually, it's probably because there's actually been barely any interaction between the two as opposed to him being faceless. Unlike the other guys (such as Sensei, Senpai, and Childhood friend), Onii-san only has a few mornings a day in which he gets to interact with Ai-Chan, so unless that changes, like she starts visiting his home after school (I know she did in the non-serialization version, but I'm unsure if that carried over here), or summer vacation begins and Onii-san takes vacation days, this is what we'll get.