I have suspicion that the mind flair is of homosexual decent. In the beginning of season 3 the mind flair takes over billys body as a host. This is very important that billy is the first victim. The mind flair later gets the lifeguard chick and then the old lady with the crazy rat. Soon the mind flair gets multiple people throughout Hawkins only to later sacrifice themselves to make the mind flair bigger in stronger. Except billy. Billy is the only one still standing not because he is the first host, but because he is attractive. Theres is no debate that the only thing billy has to offer in the show is his sex appeal. The mind flair clearly has a thing for him.

How do we know the mind flair is gay you ask? Because its an Netflix original. At the end of the third season we find out that Robin is a lesbian. Keep in mind this is at the end of the THIRD SEASON. Netflix has a fixation of added LGBTQ characters in there shows. We knew about the mind flair since season 1 (despite it not being a “major” role in the plot). Netflix would NOT introduce an LGBTQ character THAT LATE into there shows. At this point its pretty clear that the mind flair is an LGBTQ character originating from season 1.

Why did he kill billy? His one true love? Because he stood up against him and saved eleven and the rest of the gang and the mind flair took it as SEXUAL ATTRACTION for eleven. Because thats what billys character is BEST at. Attracting girls and seducing them. Since eleven was right by him, he clearly took it as that billy doesn’t like penis.

How do i know the mind flair is a guy? Because if you look carefully you can see a small penis in the battle of star-court (if you watch frame by frame). Clearly a grower and NOT a shower.

I also believe he is responsible for the AIDS crisis. But thats another story for another time.

TL;DR : mind flair wants to fuck billy