Happygun is a pirate captain from the New World who was allied with Whitebeard.
Skills and strength
The fact that Happygun is a notorious New World captain leaves no doubt that he must be a powerful fighter. In Marineford, however, his abilities were not really shown.
The pirates storm forward
He and his gang have sailed to naval headquarters with 42 other notorious New World pirate gangs, including the gangs of Doma, the Decalvan brothers, Squard, McGuy, Whitey Bay, Epoida, and Little Oars Jr, all of whom are also allied with Whitebeard, to rescue Ace from the hands of the World Government.
The enterprise failed, however, and Ace as well as Whitebeard died. It is likely that he later attended their funeral with the others.