Nothing is ever just one thing, not in life and not in writing. Everything is nuanced if you look at it long enough.
So when I say side character your mind probably goes to some famous one like Randy from Scream, or Sam from Lord of the Rings, or Jimmy Olson from Superman. Side characters are great, they add so much but they can also take away if you aren't careful. The pit that people fall into is that they use side characters as plot devices and that's a big no no, and I'm not getting into the semantics of flat and round characters, I'm talking about what I like to call warm body characters. They’re just there to be there for the character to talk to and they bring nothing to the table. I always think of Rosencrantz and Guildenstern from Hamlet, like why are they there? That’s why I love Rosencrantz and Guildenstern Are Dead by Tom Stoppard because it’s giving life in a very existential way to Rosencrantz and Guildenstern.
Your side character has to bring something to the table. They can’t just be a warm body there to be a punching bag. They need to be like your main character. They need to have goals, struggles and the like. They have to be 3d because honestly, it will probably be very lonely for your main character. Side characters need to challenge your main character in some way. I like to assign a theme to each of my side characters they can teach the main character, and your side characters have to be there at the final scene, they have to matter to that scene, and if they don't, I know killing you, darling, is hard, but you gotta.
Hear me out. There is also merging two characters together. I had to do this once because there was this cop that I really wanted to write and I had another character that was the main character’s uncle. The two had the same talking point so I forgot about the uncle half the time. So what did I do? I merged the two allowing the theme I wanted in the uncle to be portrayed in the cop. Cutting is a skill that you have to learn when writing but don't forget that merging is an option.