To unlock O´Sole Meeo in the character selection you need to kill 3000 Dragon Shrimps (the enemies that kinda looks like Onix). They first appear on Gallo Tower (Stage 4).
There are different types of Dragon Shrimps, but as far as I can tell, it doesn't matter which type you kill.
Once you killed 3000 Dragon Shrimps you can then unlock O´Sole Meeo in the character selection for 500 gold.
Happy Surviving!