That's Garf, part of the Robbo gang. I'm pretty sure he's related to Gus. He looks pretty gnarly, but he's actually pretty cool. Don't judge folks on looks!
Garf is a Craw member of Ishnail's Robbo Gang and is the brother of Gus. At the beginning of Paper Mario: The Thousand-Year Door, he and his brother Gus are beaten up by two of Don Pianta's henchmen. After the first chapter, Garf puts up a trouble in the Trouble Center, asking for someone to find the key to his house. If Mario takes the trouble and completes it, Garf will allow Mario to go into his house, but Garf will return to Ishnail's hideout.