I think Mob Psycho 100 is a good example to get into the deets of Japanese politeness just because Mob himself has so many names.
His family calls him Shige-chan. The “-chan” suffix is generally only used for girls and very young children, so this is basically a baby nickname. It’s not weird for his parents to still use it (Ritsu, naturally, calls him niisan because he is a good and proper younger brother) and even his friends wouldn’t make fun of him if they heard it. It’s perfectly normal.
His childhood friends, whether he’s still close with them or not, call him Mob. This is a nickname that probably came about when they were in elementary school, learning kanji for the first time. Or maybe a teacher misread his name once and it just stuck. It’s... slightly mean, definitely not complimentary, but it’s not weird that it still gets used. A lot of people have nicknames along those lines. Mob probably took it as a sign of being accepted, and most likely Reigen started using it because Mob said “my friends call me Mob” at some point early in their relationship.
Dimple calls him Shigeo. Now, this is an outlier, but Dimple was deliberately trying to create the idea of a close relationship between them. He wanted Mob to trust him and rely on him, and calling him by a less-formal name plants the idea that Dimple is already inside his circle of trust. Since Dimple does, in the end, become one of Mob’s closest friends and someone he trusts implicitly, it’s sort of a retroactive example. He’s also quite a bit older than Mob (quite a bit) and it’s normal for older folks to be less formal toward younger ones. Dimple may have been trying to come across sort of fatherly.
And then there’s Kageyama-kun, which Teru and most of Mob’s school friends call him. This is the most formal thing he’s called on the regular, but all of those characters are someone he only met during the course of the story. It’s normal to remain (relatively) formal with someone even after knowing them for years. Especially among boys. Even if Mob stays friends with them for the rest of his life, it wouldn’t be unusual if the only upgrade they make is to drop the “-kun” part.
So here’s our hierarchy:
Shige-chan - family only
Mob - childhood friends
Shigeo - very close friends
Kageyama-kun - new/casual friends and older folks trying to be friendly
Kageyama-san - complete strangers or people being intentionally formal
