A lot of my characters have pretty long reasons behind their names, I think it's a lot of fun trying to fit in as many different criteria into a name especially since I have a very hard time coming up with names, it helps to narrow down the list of possible names
This got really long so my explanations are under the spoiler:
My most well developed characters (at the moment) have nicknames and real names, I got Billie and Sunny's names from a movie I watched which was a Western and two of the main characters were called Mister Sunshine and The Kid which I though would be fun to reference in their nicknames since Billie is the youngest character and Sunny is tall and charismatic like the character of the same name, Santiago got his nickname since the character he's mainly based off is called Cowboy, and I thought that name suited that theme, but it also means Saint James so I used that to pick his real name, Yorkie's got a werewolf theme going in his character design and his name was picked by his other creator it suits him since he's the one English character out of the group and Yorkie is also a breed of dog, unfortunately Leo and Marlin's names aren't as well chosen, I initially picked Leo since I like that name, it's got a zodiac theme which suits the 70s setting and it's also related to lions which are usually associated with leadership but it feels a bit too on the nose, and for Marlin's name, I wanted an animal theme and it was also viable as a regular name but I don't think it really suits him and I think the fish theme kinda kills his cool tough-guy vibe lol
For my Cyberpunk characters, Donnie was picked since it's the name of a prominent Cyberpunk character, Costello was the name of a character that his face claim played (And I think it's a cool name lol), Marcus has a much longer explanation, I wanted his initials to be M E since that's also the name of a song by a musician who inspired his character about an AI that turns evil, the song was inspired by the book "I Have No Mouth and Must Scream" and the last name of the author who wrote that book was Ellison which I picked for him, and Marcus was picked since it comes from the word Mars and the TTRPG I made him for is set on Mars (And it can also be shortened to Marc, the first name of another musician I like) and in-universe the last name of the owner of the robotics company that made Marcus is Ellis, so it would be like a robot character calling themselves Terra Skynetson which I think is a funny joke