Allen O'Neil is General Morden's most trusted soldier. During the Regular Army's operation in the Käthehirt Valley to destroy Rebellion motor pool, he singlehandedly slaughtered the incoming Regular Army troops as they climbed up the mountain. He taunts the player with the phrases "C'mon, boy", "Go home to mommy", and "you're mincemeat" (or minced meat), as well as maniacal laughter. Upon defeat he always utters "See you in Hell!" He uses an M60 machine gun, a knife, and blue grenades that erupt in an angry red flame upon detonation. Despite the fact that Allen clearly meets his end at the conclusion of every encounter, usually in a gruesome manner and on one occasion being devoured by an enormous Orca, the game makers have humorously brought him back in each new iteration of the series. An interview with the game-makers, which is unlockable in Metal Slug Anthology, reveals that the only thing that keeps Allen alive besides his muscles and guts is his devotion and will to return home to his wife and son after a hard day of fighting.
Allen aids the player aboard the Mothership in MS3 (unless the Metal Slug employed earlier is still surviving and in current use), laughing all the while. It should be noted that despite how strong he is against the player(s) in MS3 and before, his strength is much less on the Mothership than usual. One burst shot from his M60 cannot kill a clone by itself. He also has less "Life", and stands still against the hordes of clones. Thus, it does not take much for the clones to finish Allen, even by melee attacks. Because of this vulnerability, it can be assumed he's more of a decoy while the player assaults the clones.
Allen appears for the final time in MS4 as a boss in Mission Two and during the Final Mission, but in actuality this Allen is only a Terminator-like android clone built by the rogue scientist who is the main villain in MS4 and not the "real" Allen. Using a similar process, the same scientist was successful in creating a mass produced Robot Morden Army that led people to believe Morden was behind the Amadeus Syndicate. Possibly to frame him for the scientist's crimes. Unfortunately these mechanical incarnations were even weaker than their flesh & blood counterpart, having the weakness of falling after a few well placed shots from a simple handgun.
Role in Super Smash Sisters
Allen O'Neil is an NPC helper in Super Smash Sisters. He can sometimes use his M60 machine gun to help girls in battle. When someone is too close, he uses his knife to chop his opponent up. He can his grenade on high ground but can now throw them on low ground. He can still try to mess with the opponent girl's mind like he does in the Metal Slug series.