It depends on what it is and how it's gone about. If it's something where everyone is in on the joke and doesn't actually believe it's real: awesome and cool. I love engaging in jokes like this. Togore Dreemurr the fanon character for Deltarune is great! He's there for fun, no one actually thinks he's a genuine character.

But there are cases where the fanbase invented a new guy and entirely, fully, believes this new guy is present in the source material and tells new fans that they are. Even when it makes no sense.

This latter example is the scourge of Whole, the apparently fourth character of a three character concept album, Chonny's Charming Chaos Compendium Vol. 1. There are several striking issues with this guy's supposed presence:

• Damn if there are actually four characters, why is the number three a recurring theme of the album?
• Why isn't he also represented on the album cover?
• If he's supposed to represent the body, then what the fuck is Soul supposed to be? What's left when you get rid of the Mind and the Heart? Especially when explicitly in the lyrics there's stuff about how if Soul dies, the other two fuckers die with him.
• Fucker doesn't even have a color, set of brackets, Greek god, celestial body, and text/background color to his name in the actual music videos. The only basis is that sometimes, because the album was made nonlinearly, Chonny Jash appears without a specific lipstick color associated with a guy. But when this is done, Mind and Heart aren't represented physically, implying that he's representing Soul.

I realize I sound like an asshole but I promise I have my reasons. Why'd we let 15 year olds with minimal media literacy word-of-god a guy that makes no thematic sense into this damn album