She's basically like a pet for him. Like a beloved big dog that welcomes him back home after a day of work and keeps him company, makes the apartment feel more lively. I wonder if he's actually going to try to fix her life at some point. Sending her back home eventually isn't necessarily the correct answer either. Having her go to a local school might be better, although perhaps the official business involved would be too difficult to deal with since he's not her guardian. One further complication is that the Japanese aren't adults until they turn 20, so she still has a long time to wait before the guardianship issue is removed. In the West (and in the near future Japan) it wouldn't be nearly as long since it would only be until 18.