My best friend and I in elementary and middle school were classmates. We would hang out outside of school a lot, doing whatever boys who were like, twelve, did in the early 2000s: I have memories of playing Playstation, watching The Ring, downloading music on Kazaa after logging with his dial up connection, skateboarding, going to movies, eating dinner with his family. It was a close friendship, and we shared a lot with one another.
In retrospect, however, this person was a narcissist and sociopath, one who seemed to enjoy playing people off one another and watching them squirm. Even while being my friend and hanging out so often, opening up to me, he'd exclude me from the crowd he commanded at school and undermine me, just a little at a time, with small digs. A mutual friend of ours had a really interesting hobby that I started exploring too, and he mocked me incessantly for wanting to imitate him. He also painted me as "uncool" because of some other, more nerdy people I was friends with. Then, he'd become friends with them to try to take them from me. It was a classic example of isolating abuse, with the goal of him being my only consistent friend. I think he got off on the control, really, being in the position of authority who could dictate what was what.
The collective mystery and social taboo of sex at that age was the backdrop, and also the best example, of this. While we were all discovering what sex was at that time, he was the youngest of three siblings, so lorded over his limited knowledge. But the worst of it had to do with sexuality. I'm a straight man who tends to present feminine. Back when calling people "gay" was an insult (in terms of absolute time, and also at that time in life) — and an understanding of the differences between gender identity, gender expression, and sexuality was less in the mainstream — he would openly question my sexuality constantly. If I told him instead about a girl I had a crush on, he'd make fun of me for it, and tell everyone. Teasing about crushes is pretty normal, but in this case he'd reveal the secret when it advantaged him. Sometimes he would keep my confidence. Other times, not so much. He'd turn what I'd said into a fetish that rendered me perverted. The felt result was that any public expression of sexuality was consistently threatening to me. It got particularly bad in middle school when he looped a third person in on it, who I suspect was insecure about his own sexuality. I knew even then that I liked women, so I don't remember having a crisis of identity of my own because of the bullying. But it was frustrating to deny being something that I wasn't, day in and day out. I didn't want to go to school anymore.
TL;DR My "best friend" in early adolesence deliberately and slowly eroded my sense of self-esteem. It's taken me a long time to build it and learn how to be masculine and sexual in a non-anxious way. While I now have a ton of language to use to understand what he was doing, I'm still not sure that I completely understand why. I think he was a thoroughly unhappy person. He would later die in a drug overdose, which was a whole new set of emotions for me.