This might just be a general Japanese-Language question, but I noticed, that Iida calls Ochaco with the honorific "kun", instead of "chan", or "san" for example.
Why is that and what does it say about Iidas character?
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From what I was able to find out, -kun, while mostly used for males, can also be used for females. According to this,
Kun for females is a more respectful honorific than -chan, which implies childlike cuteness.
-san can also be used but according to the same source as above,
Due to -san being gender neutral and commonly used, it can be used to refer to people who are not close or whom one does not know. However, it may not be appropriate when using it on someone who is close or when it is clear that other honorifics should be used.
I think this just means that Iida tends to be formal, or even more formal, towards females/girls (since -kun is informal when used among fellow boys, but despite this, he still acts serious even with his male classmates) and might be just his way of showing respect.