Prinny is the exploding penguin. They are a 1 Star critter that you need to get a LOT of to fuse together to rank up other characters.. so you end up with a LOT of prinnies in your character inventory. Especially if you aren't paying attention.
it takes 6 of the 1 stars to make a 2 star prinny. then 6, 2 stars to make a 3 star prinny.. max star is 6 stars. When you do your gacha pulls, the highest rank are 4 star characters so you need to rank them up to 5 then 6 star characters... sooooo.. that's a lot of prinnies.. >-> and they just FILL the character inventory

Well the japanese version eventually added a special room where the prinnies get stored in when you get them from missions. (if you get them via rewards/mailbox they go into the inventory) This special room is SOOOO nice.. and you can rank them up in there too.. though if you rank them up, the ranked up ones go to the character inventory afterwards.. o3o so make sure you have enough to rank them up all the way. (room only ranks them up to 4 stars.. so you'll have 4 and 5 star prinnies sitting in your character inventory until you rank up your main characters. o3o) (the rank up is called awakening)

then there is the evilty unlocking where you feed your duplicates to the ones you're keeping to unlock skills and stat buffs. o3o you need 10 dups to fully unlock these. Though you don't need dupes, you can use rainbow prinnies or 100 prinny husks per unlock. o3o this is called N.E.

Luckily rainbow prinnies and prinny husks don't take up character space like normal prinnies ovo