Although lots of people like to mess around with custom characters in RPGs, Persona 5’s protagonist actually does have a canon name - in fact, he has two different ones. There are valid arguments as to why Joker’s real name is both Ren Amamiya and Akira Kurusu, although technically speaking, one of these is more correct than the other.
If you called your Joker Ren, good job. You chose the name that many people believe is the true canon one. If you called him Akira, that’s fine as well, although a) it’s not necessarily right, and b) come on. Akira Kurusu? You can’t get any more on the nose than that.
Akira Kurusu is Joker’s name in the Persona 5 manga, which is why certain people have been led to believe that it’s his canon name in the game. However, Ren Amamiya was used in the official anime series based on the game, as well as in Persona 5 Dancing in Starlight. The manga is outweighed by every single other piece of media based on Persona 5, meaning that you can obviously choose to base your game on it, but it seems as if Atlus and any others companies affiliated with Persona have largely chosen not to.
Outside of the fact that Ren Amamiya is used far more often in Persona 5 media, it’s worth looking into what each name actually means. Ren means “rain,” and likely refers to the fact that it was raining when Joker met Ryuji at the beginning of Persona 5. “Amamiya” means “Palace,” and is derived from Joker and Ryuji’s joint discovery of Kamoshida’s Metaverse dungeon. It’s a nice way of basing Joker’s real name on the events that occurred at the start of his story - it’s as if he was inadvertently named after a destiny he would one day realize. And so, I’ve named my Persona 5 Strikers Joker “Ren Amamiya,” which, when roughly translated, means “Rain Palace.”
Do you know what Akira Kurusu means? Killer Kill. Not just Killer, and not just Kill - it literally means Killer Kill. I think that’s a bit less nuanced, isn’t it? Also Joker doesn’t even kill anybody - none of the Phantom Thieves do, that’s the whole point. Imagine Okumura is like, “Who are you kids and what are you doing here?” and Joker just goes, “Give up Okumura, it’s the end of the line - Killer Kill is here to take you down.”
All things considered, not only is Ren Amamiya the canon name based on its multimedia usage - it’s also just a lot more elegant and tasteful than Akira Kurusu. I know we all love Akira and the idea of naming Joker after the film might seem enticing at first, but if you’re here because you’re trying to figure out what to call him for your first ever full canon playthrough… Well, all I can say is that you should really choose Ren Amamiya, unless you want to tell Sae Niijima that you’re called “Killer Kill” during your interrogation about literal murder.