Well, at the end of the day, I find Touma a little dull, with no special salient trait.
All the girls around her are more interesting.
Honestly, Touma's main personality trait (besides being cheerful and outgoing regardless of the circumstances) seems to be that she's a complete idiot: she consistently fails to know or understand pretty much anything, even things you'd really expect her to. It's just that everyone's too charmed by her to notice. It's why I haven't been as harsh on her as some folks in the comments; I don't think she's being manipulative, I think she just acts on instinct and genuinely doesn't understand human relationships well enough to realize what effect she has on people.
THIS
Touma is not the point of this series. Touma's effect on the other girls is the point. I think it's very much the idea of the series that Touma is essentially blameless. Touma is basically sex itself - in fact, the near ideal of no attachment, no consequences, gentle, consensual sex. I think Amano Shuninta is playing with the effects of this on the characters, how it intersects with life and love. Touma represents different things to different people, but Touma is basically just Touma. Sex is just sex.
Sex and lust don't follow rules. Even consensual sex. It doesn't respect your person or status. What you desire doesn't respect what you intellectually think you SHOULD desire or how you think things should be. It's just there. And it completely disarms everyone, strips them bare.
I think Shuninta writes very well and just has a bad reputation here because a lot of people have hugely idealized visions of love and sex, and think it should follow rules that it just doesn't. But I like her characters, even the ones I don't "like".