Okay, this is gonna be a steaming hot take, and honestly, I've been kinda scared of sharing it for multiple reasons. Mainly because I know a lot of people really like this specific headcanon. Hell, I thought it was cute at first myself. Key words being at first. But really looking at this, I don't think it's plausible. What is it, you may ask? It's the idea that BF (from FNF) might be Hatsune Miku's brother.

Before we get into my main issues with this, I'll just say it's not an unfounded claim. The FNF devs have joked about the possibility of this being canon. However, they have yet to make this the case. But considering the main problems with how BF could be related to Miku, I actually hope they just leave it be. And here's the main reasons why I have a personal issue with the matter:

It takes away BF's agency of being good at rap battles.

Now, this point might be confusing to read at first glance. But to put it simply, BF doesn't have to be related to Miku in order to be good at singing. The main reason he takes it up is so he could impress his girlfriend, and it seems to be a hobby both of them enjoy. Tying this to him being related to Miku just makes it come across as "oh, he would obviously take up the handle just because his sister is one of the most popular music voicebanks in the world", which in my opinion, would take away a lot of BF's strengths and would act as a cheap explanation for the one character trait he has.

Not everyone who's good at singing has to be related to a popular singer. They can pursue their career without having to be tied to someone who's already leagues above them. If BF was Miku's brother, that would imply the sole reason he's good at singing is because of his sister just happening to be a very popular singer, and it just makes BF's skills less of a genuine hobby and moreso something he had to take up because of him being in Miku's shadow, which would make FNF's gameplay pretty depressing in hindsight.

Copyright

Correct me if I'm wrong on this, since I'm not entirely sure how Miku's copyright works, but I've heard it's one of the reasons the FNF devs haven't canonized BF and Miku's relationship. They would have to get permission from whoever holds the copyright to Miku to canonize this. And it's not gonna be as simple as asking a question, since FNF isn't gonna be a small game, if the kickstarter is anything to go by. She appeared as a cameo in Week 5's background, but that was barely noticeable. Giving her an active role, on the other hand, will complicate matters, especially if FNF branches out to the point where this copyright on Miku will have to be addressed.

Controversy (cw for false accusations and pedophilia mention)

Yes, there was controversy relating to Miku and FNF... sort of. In case you don't remember, in late December of 2023, Ninjamuffin was "exposed" for liking NSFW art of fictional characters on both his Newgrounds and Twitter account. This was after he made a pretty immature joke about the situation regarding Palestine (which he later apologized for), but nonetheless, people immediately started labelling him as a predator and whatnot because he liked artwork of underage characters, like Hat Kid from A Hat in Time for example, and Miku herself (although I'm pretty sure most of the art featuring said characters had them aged up). Now, I could understand why people would be upset with it, but bear in mind, these people were calling or comparing him to Epstein, an actual child predator, over him liking artwork of Hatsune Miku. You heard that right: they tried to frame him as an actual predator for liking artwork of one of the most popular fictional characters in the world. You cannot make this shit up.

With all of that in mind, I highly doubt the FNF devs will ever canonize Miku and BF being related because of this. I know for a fact that it's just gonna cause more drama, and if I were Ninjamuffin, I don't think I'd want one of my characters to be associated with the character people actively used to slander me left and right (granted he's not entirely innocent of his actions, either). I know this happened months ago, but it's something I can just never forgive, soley because the FNF fandom deemed this going "too far" despite that fandom being responsible for holding actual groomers and predators in it.

(By the way, fuck the FNF fandom. Just because not everyone there is like this, that doesn't automatically excuse how bad it is, but that's for another day.)

Overall, while the idea of BF being Miku's brother is cute, I'll admit, it just wouldn't work out in the long run to me. I'm not saying this headcanon is bad or that people shouldn't support it. I'll even say that it has some potential to be good if the FNF devs want to pick it up. But the way it frames BF's hobby as something he's only good at because of him being related to Miku, the copyright issues regarding Miku's use, and the controversy surrounding Ninjamuffin that happened and how people used Miku to slander him, I'd say it's better to be a fan interpretation rather than be implemented into canon. And this is coming from someone who jokes about and actually supports the idea of Nene and Samurai Asshole being related, but I digress. At least that isn't hard to work around or has massive gaping holes.