The rival.
Especially the douchebag Machiavellian type rivals that the main character ends up friends or teaming up with after the main character 'proves their worth' to them by being stronger. No character growth outside of now they work with the protagonist.
I really want this trope to be subverted one day, like the douchey rival shows up and everyone is just like "Wow what a dickhead" and just relentlessly bullies him until he goes away.
It used to be that the rival character was just... some other dude, whose personality clashed with the MC (if the MC was hot-blooded, the rival was cool/calculating and vice versa) and basically be a source of drama/narrative tension, with each characters narrative arcs being to gradually realize that they were basically on the same page, and to overcome their differences to work together and beat the bad guys. But while the generic shonen MC's character has been relatively static action cool guy (the biggest shift is that shonen MC's nowadays are REALLY REALLY REALLY empathetic and emotionally open/in-touch), Rival character's just have gotten more and more extreme in order to be memorable... so you get ridiculousness like Accelerator from Index and stuff like that.
Although now that I think about it I can't think of any recent shonen outside of My Hero Academia that even has a rival character anymore. It's all usually Trio's (hot-head/cool guy/girl) nowadays.