A Dos and Don'ts Guide to RPG Mascot Characters
I mean, besides just "don't". That would make for a short list. Let's assume that a mascot character is like getting 85 or more on Metacritic: The designers don't get to eat if there isn't some adorable chirping nuisance making the game ever so slightly worse.
That said, there are plenty of mascot/pet characters in RPGs (mostly the Japanese kind) that can turn out genuinely pretty neat. The trick is to follow the rules below: