|Because I'm not sure if people would classify some of this as spoiler or not I'm just going to be safe though most people perusing this have already read this in most cases.
Spoiler (highlight to view)
I personally saw Satsuki as the original character put into place to spur Aya into action or to at least influence Aya into acting in confessing her love for Junpei before anything with Tsukasa could ever be reestablished since she was always pushing Junpei around and whenever he ran into Aya it always seemed to be a nice reprieve for him. Yes a lot of times he felt very comfortable with Satsuki but at the same time there were times when he also felt very pressured with her always forcing herself on him. It of course doesn't take much to figure out that Satsuki was more of a throw away character that was there to simply make Junpei's choices more difficult and to keep things from being simple. She also of course was perfect for being the fan service that is so popular apparently in shounen harem romance. I myself am not a fan of her type of character because usually her type is cast simply to show off as much as possible in a shounen manga and becomes distracting, annoying, and detracts from the story in general. Satsuki though is usually more bearable at times since she can also act as a pick me up for Junpei when he was feeling sorry for himself (when is he not feeling sorry for himself?) and inadvertently pushing Aya and Junpei together. It appears to me though that towards the later part of the manga Kawashita realized that Satsuki wasn't a strong enough to push to really put Aya and Junpei together and so comes what I believe to be the reason that Mukai was introduced to be a much stronger, more direct push towards JunpeixAya. Did Satsuki get the short end of the stick? Yes, she did, no real argument there since its accepted by most in some way, shape, or form. I think I would have liked her character a bit more if she was less of a friend and more of an actual push in the relationship and hence the need for a character like Mukai to give a great big push in the relationships. I like all of the characters with the exception of Chinami but Satsuki wasn't necessary once Mukai was introduced since she was simply doing what Mukai was through more questionable methods, intentionally, and usually ending up ecchi (although mukai usually came to the same ecchi result too -_-). Sacrifices must be made and Satsuki was one of them.
If it makes sense, your probably wrong.