How tf did he spin this into her having a superiority complex?
Wouldn't it be more natural to suspect that she's hanging out with you because your mom asked her to? They know each other and she came when your mom pushed you out of the house to get milk.
I've tried to understand the logic behind these types of characters and the logic behind the author's writing them. The only conclusion I could come to was these types of characters/people fitting into the shoes/roles imposed on them by their social environment, either directly or implicitly.
For example, it is common for these loner types to have a background of being late to the entrance ceremony on the first day of school, which is an event where students build their cliques, and from how it apparently seems in Japan, it isn't easy to squeeze into an established group, despite it being easier to get kicked out of one. Not being part of a group, is essentially being a part of a group of "outcasts". Whether you have the personality to excuse yourself into a group is what determines if that's the pair of shoes you will be expected to wear for the rest of the school term, since that reputation will persist until graduation and extend past it if your future is connected to those with a shared history who might bring up the past.
I suppose the reason this character acts the way he does is as a defense-mechanism; instead of feeling rejected by the world, he rejects the world instead. He projects a superiority complex onto the FMC because he sees her as superior (her role) and himself as lesser (his role). This is the reality he imposes on himself as a result of his perspective shaped and marinated in his experiences from school. The defense-mechanism is at play when he frames it in a way that makes
her look bad. It is an effective way to protect himself from the negative thoughts and justifications that might come from isolation, but as we can see, it hasn't done him any favors.