From the Super Mario Wiki
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Koover (called Tātasu in Japan: タータス) is a Koopa Troopa who spends most of his time hanging around in front of the Koopa Village Toad House during the events of Paper Mario. Koover is also fishing friends with Goompapa, Muss T. and Fishmael.
Mario and Goombario first meet up with Koover when they initially travel to Koopa Village, and he greets them. He also warns them of the Fuzzies who had recently been attacking the village before getting his shell stolen by a Fuzzy. Though Mario will receive no reward for doing so, the plumber can whack the Fuzzy who committed the crime with his Hammer to regain Koover's shell.
He is very minor in the game and doesn't have any big role. However, he does partake in a long chain of Letters Mario and Parakarry can optionally deliver by first receiving a Letter from Muss T. and sending one of his own to Fishmael, who writes back to him. Koover then has Parakarry give one of his Letters to Mr. E, at which his role in the Letter relay ends. Earlier in the game, though, Koover also sent a Letter to Kooper through the Post Office, revealing that the two are friends.