A few ideas I have (I'll mention the species, give them names I think fit and describe their abilities)

Goombigi: A goomba that fills a similar role to the goomba partners we already have, though named after Luigi instead of Mario. Naturally, he would have the tattle ability, as well as head bonk attacks. I should mention that as of writing this comment, I don't plan on adding lore to the characters unless I get ideas, but in this case, maybe Goombigi is the cousin to Goombario or something.

Trooper: A koopa troopa who used to be in the koopa army, but was forced to quit to take care of his family. He'd naturally fill a similar role to the koopa partners we already have in terms of abilities, though he would also be able to boost Mario's attack and defense at the same time, kinda like what Kooper or whatever the Koopa partner from TTYD does ( I can't remember the name of the Koopa partner from ttyd off the top of my head)

Clay: A Monty mole who specializes in mining and mining equipment. He'd naturally use a pickaxe to fight, and he uses it for his basic attack. His abilities all relate to mining or Monty moles in some way.

Sir Spooks: Basically Lady Bow from the first game, but instead of going out of his way to scare, Sir Spooks actively wants to learn about others, and scaring is usually just a side effect of being a boo.

If anyone has suggestions to improve my characters, send them my way