Kana is totally unromantic, so Fujioka has little choice but to make himself feel like a part of her daily life and hope for the best. At least he's making Chiaki happy by functioning as a sort of weird father figure for her.
@SuperOniichan Kana is that, but it's not really relevant to the total absence of any romatic sense in her. After all this time it has never visited her mind Fujioka might be interested in her. She has wondered a few times why he keeps coming back to their apartment. She never questions the numerous female friends (including the one miserable crossdresser) of the sisters who come to visit (that I can remember). It's just Fujioka who keeps appearing that baffles her, so she doesn't even consider him a friend! It's obvious the Minami household is generally very open to friends, but in Kana's mind that courtesy doesn't extend to poor Fujioka.