Rin Hoshizora is Transgender

Rin was made fun of when wearing skirts as a child for being too "boyish" for them. (Click to go back)

Difficulty Expressing Femininity
One of the main reasons Rin doesn't want to wear her dress in "Love Wing Bell" (S2-5) is because, as a child, she was made fun of for wearing skirts by her classmates. Many times, when a child expresses their identity in ways outside of the norm, they're bullied for it. It certainly seems this was the case with Rin.

Her expression when her classmates "catch" her wearing one on the way to school is one of resignition, which then turns to embarrassment. She expeceted this; Cisgender girls often don't have this issue (though that's not to say it never happens; more on that in a bit).
(I am entirely overanalyzing but that's what makes this fun, right?)

Many could say that it's simply because she has short hair, but I honestly think if that were the case that she would just grow it out. Being told she "looks like a boy" clearly causes her a great deal of distress, even at that age, and if the hair was the only issue I feel it would have been easy to overcome it- Rin doesn't seem all that attached to her hair, after all.

Personal Connection
When I was in elementary school, I began to have Gender Feelings; the overwhelming urge that I was not supposed to be a girl. I did not yet have the language or support to describe these feelings.

As such, I stopped wearing skirts and dresses altogether. I felt so wildly uncomfortable expressing femininity that I rejected it altogether. As such, years passed, and when I did try to wear a dress, or a skirt, or wear a hair clip; I was laughed at.

It was (I think?) mostly good-natured, but the teasing I dealt with for trying to break back out of my shell was unbearable, and I soon found myself in a similar situation to Rin- I didn't want to wear a skirt or a dress in public anymore because people acted like it was wrong for me to do so. This would affect my relationship with my gender for many years.