I also know nothing about borderline personality disorder, so I can't add anything to the idea that Io may or may not have it. However, there are certain aspects of Io's personality that are established, and based on those I feel like her behaviour makes sense. Bear with me, I know a lot of this is a rehash of what most or all of us already know:
1) Io thinks she has no real worth as a person - she's stated this many times.
2) She doesn't enjoy being around, or interacting with other people in most cases (Sensei and Uta being the exceptions we're aware of).
Now, I believe #2 to, partially, be an extension of #1 - After all, if she's worthless, why would these other people *want* to interact with her, why would they want her around at all? And if they're saying otherwise - aren't they likely lying about how they really feel? Why hang around people who are lying to her all the time? And similar thought spirals caused by extreme low self esteem.
Why are Uta and Sensei not lumped in with everyone else? Uta I don't know, it likely has something to do with her, and Io's shared past. Sensei it might have to do with the fact that he *isn't* a good person, and Io may understand that latching onto him like she has will wind up going badly for her in the end - which, since she's terrible, is a fate she deserves.
She also likely has a healthy dose of being an introvert, which means that interacting with people in a social setting, even if she's enjoying it, can be incredibly draining, which adds to the desire not to be around them.
Largely I just don't know why she things so poorly of herself - and I think that just hasn't come out in her story, yet. When it does, it might help explain the exceptions, better than my guesses. Either way, I'm sure her isolationism, and self-destructive tendencies stem from her personal view of herself to a large degree.
Just my pair of copper coins for the discussion.
