Koopbeard is a Koopa.
Koopbeard was an ordinary Koopa until he ate the Beard mushroom. The same thing happened to other members of the Mushroom Kingdom Beard Appreciation Club.
Koopbeard comes from a poor family of Koopas. His father, Koopie Kooper, used to be an underling for Bowser but was disgraced and ran away with his family. Jacob, Peach's brother, helped them find a home. Since then, Koopbeard and his family adore Peach and Jacob for their kindness, even hanging up a portrait of each of them in their home. Despite being poor, they live happily together.
Koopbeard came across the Beard mushroom when looking for food for his family. He brought it back and they made a fricassee and so they all ended up with beards. They found that very funny and laughed it off.
Koopbeard looks like a hipster and you would believe he is one until he comes out of his shell. He actually has a heart of gold and all his friends can count on him despite his snide remarks. He has a secret crush on Claudia.