Way back in the old MotK boards, I used to be one of the people that would change my avatar every few months, usually a different Koishi image. But then one day I decided I want to stick with one going forward, just like some of the other notable members at the time.
I use the one I have now because another member decided to make this one for me (not the artist, just made this image crop and all, the original artist is
B-ginga btw, but don't look up their recent works though, it's all NSFW), and I was super grateful for it. I liked it enough that I decided this was the avatar I was gonna stick with going forward. I liked the casual look of Koishi from it.
I still have the DM of this exchange in the old board funnily enough.