Unrequited love is one of the more difficult romance tropes to handle. On one hand you have a relationship between two characters where feelings are very one-sided and seemingly go nowhere. On the other you have a chance to show off a reason why there is this rebuff. Like in real life, not everyone in a story gets a happy ending nor does the hero always get the girl or guy as the case may be. One of the any ways I've seen this is by having the character who is on the receiving end of the affection be some sort of cosmic plaything where they are doomed to some inescapable destiny of doom. While this certainly explains it, it's almost like you avoid the issue by making it a complete non-start.

There are countless ways that this can be done without detracting too much and without having to resolve the relationship by having the object of affection slowly come around. Take this for example. Five adventurers set out on a trip. Kate is over heels for Clark. Clark is pining over Claudia. Claudia however wants Kate. Violet and Pine are twins and can't pick who out of the other three to go after. Now, all of them are aware of the attractions and Claudia as our example only has eyes for Kate. The other three might as well not exist for her as romance options. No matter how often Violet or Pine say something to her, she'll be gentle and say no to it. You can keep this dynamic right on through even if Claudia doesn't get Kate in the end. The one sided nature of this can actually spur your character to act rashly in front of the object of affection ad maybe even sacrifice themselves to save them.

This particular relationship is difficult because we want a relationship to be a two way street. We love seeing characters slowly fall for each other. So when we run into this type of relationship where the love is heavily one-sided and not returned, we wonder why and maybe even write fanfiction about these characters where they don't have these hang-ups.

The thing to remember is this. Real life love goes unrequited all the time. There's no rhyme or reason as to the fine details of why or how as there are many explanations. Having a character experience this may allow you to bring someone into their life that's willing to return their affection. Then you can pull them slowly together and go from there. Equally reasonable is to just let it happen and stay that way. After all, the hero doesn't always have to get the girl at the end.