I'll answer this in two parts, first the first one I ever even vaguely have memories of from quite early childhood, and the second the more "normal" answer.

The first I have even a vague memory of? It's a surreal answer. Noble from Thumbelina: A Magical Story. A sugar fairy who Is Gender. I was very very young. I was just drawn in by things I felt were "magical" back then. Something about Noble stuck with me forever.

The first that was more of an actual crush? Sailor Mercury. I found everything about her very attractive and interesting. I wouldn't say I still have a crush on her but I do still like her as a character a great deal and I probably aspired to be like her in life quite a lot. I wonder if I succeeded or not.