In fact, since seeing that this would be one of the prompts, I have racked my brain and come up empty. I can't think of any fictitious character I would have dated in high school. This is largely due to two reasons. Reason the first: I don't read books about high schoolers. Never have and the prospects of my beginning to do so aren't high. Reason B: related to the first reason, I would not have, in high school, dated someone outside of that age range.
Now, I suppose I could make something up. I could pretend that there is someone or I could do something like pick Arwen because with an elf there's going to be a ridiculous difference in ages no matter what you do. But I'm restrained by the idea that, not only would I have to be or have been interested in dating the young woman in question, but honesty compels me to consider whether or not she would have been interested in me. The odds of an immortal elf princess finding me irresistible seem... long. To say the least.
As a result, I don't actually have an answer for this one. In my defense, allow me to say that the young woman I did date in high school was wonderful in all regards and I am quite pleased that she later consented to marry me. We are rapidly approaching the point at which the majority of our acquaintance will be after the marriage instead of before it and, were I to given to decide now, I would marry her all over again without hesitation. If not exculpatory, then I hope that is at least explanatory.