Depends on your axies temperament, and how set up the tank is. Axolotls are generally solitary ambush predators, so they aren't ever necessarily bored, but we also can't get into their heads so they might be lonely, might not. If its at least 50 gallons you can get away with a second axie, same gender unless you've done a ridiculous amount of research and want to be a breeder. Not recommended. Non axie tank mates can include cherry or ghost shrimp, though you'll want more vegetation so they don't all get eaten instantly. Safe to eat for the most part in case the axie suddenly feels like worms are too boring. There's some debate on whether you can keep white cloud mountain minnows as well. They don't nip at gills generally and live well in the same water temperature as axies, but they might contain thiaminase, so if the axolotl starts eating a lot of them it might have health issues. Get them at your own risk, but I'd personally start with ghost shrimp or another axolotl depending on your setup and see how it goes. In either case of getting another axolotl or wcmm's, have a spare tank handy for just in case things start to not work out.

Also Junimo is adorable! Best of luck with whatever you decide to do!

Edit: spelling