This boss/mini-boss appears in most of the Metal Slug games. Most of time he comes at you with huge machine gun he carries one handed but he always make sure to fight you shirtless, definition of a badass
Fights you outside while climbing up the mountain level
Metal Slug 2: Super Vehicle - 001/II
Fights you on the bridge in the artic level. After he is deafeated he falls of a cliff and and gets eaten by a killer whale.
Fights you while piloting an airplane and is dropping bombs on you. After he is defeated he later helps you at the end of game when you're escaping the alien ship .
Metal Slug 4
Allen appears in the second boss fight, controlling the tower pillbox. You fight him again in a factory, after he was he is defeated, his skin tears away revealing a robot, starting a second round of the fight.
Metal Slug 7
This the first time you fight Allen as a end of level boss as well as you get to fight him while both of you pilot Slug Giants.