Pigma is a fat pig with a lot of squeal. He was a member of the original Star Fox Team back in the days of James McCloud as leader and Peppy as his right hand man. Pigma betrayed James to Andross who killed him, Peppy only just making it out alive to retell the story to Jame's son (and his future best friend) Fox McCloud.
His destiny then lies in joining a new team, considered the "Anti Star Fox" named Star Wolf. Joining this team only went on to rub salt into the wound for Fox and Peppy. He likes working for the evil side because it involves a nice big paycheck and Pigma is the embodiment of greed. Due to is hated of his past team, he has a thing for hunting down Peppy in the heat of a battle.
Eventually though Wolf O'Donnell, leader of Star Wolf, kicks him out for being too greedy and unworthy of his trust. This infuriates Pigma who not only goes on to try and control the Aparoids, he also fuses his body with a ship to become a powerful threat.
 Star Fox 64
Pigma joins Star Wolf in a big for wealth and power, still pumped from the killing Fox's father. He always goes for Peppy in the heat of battle, trying to kill off the last member of the original Star Fox. He attempts to make jokes and put people down, though he ends up coming off as pathetic.
 Star Fox: Assault
Having been expelled from Star Wolf, Pigma is still trying to find a way of becoming rich and powerful. This is when he attempts to control the Aparoids for his own means, but fails and goes on to fuse with a spaceship.
 Star Fox Command
From minion, to freelancer to boss. Pigma comes back as a massive threat. Encountering him isn't guaranteed though, depending on what decisions you make during the game.