"See you in hell..." - Allen's famous last words.
Allen is infamously known as the "Immortal Devil Sergeant" and "Wolf Sergeant", as even though he is constantly killed every game, he manages to survive every attempt on his life.
Allen approaches the battlefield in a hot-headed and insulting manner when it comes to fighting the soldiers of the Regular Army. Though Allen is capable of being a high-ranking officer, he prefers to remain as a sergeant, because it is the highest rank possible which allows him to fight in the front lines, which he loves taking part in.
His sheer strength is matched by far of his ego, often slagging and making comments about his enemies. While his superiority complex does get the better of him, he is an adept soldier, capable of enduring more punishment than his weaker comrades.
Allen O' Neil was the most trusted soldier from Donald Morden while he was a commander in the Regular Army. When Morden started his rebellion, Allen followed him out of loyalty, helping him in his coup d'état against the Regular Army.
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 was ultimately defeated, and believed to be dead.
Allen reappears again to fight on the bridge to the Rebel Secret Base in the Artic. He will always have 2 rebel soldiers to help him in the fight. As he was defeated, he fell off the cliff and was eaten by an Orca.
Allen is the first mid-boss of Stage 5, controlling a Hairbuster Riberts, and was defeated again. Later in the same level, when the heroes are making their escape from the crumbling Rugname, Allen shows again, this time to give support fire for them, but only if the player is not in possession of the Metal Slug.
Metal Slug 4 : Mecha Allen
Allen appears in the second boss fight, controlling the tower pillbox. After the tower is destroyed, he is thought to be defeated. Notably, this is the first time in the series that Allen serves as an end-of-level boss, as opposed to his usual mid-boss status.
The commandos fight him again in a factory, though he is revealed to be a robot created by the Amadeus Syndicate, which was also responsible for creating Morden Robots. The presence of Morden Robots led the general public to believe that Morden was also the leader of the Amadeus Syndicate.
Allen himself does not appear in Metal Slug 6 but some of his characteristics make an appearance in the final mission, when you fight against either of the other main characters. They share some characteristics with Allen, such as his grenade throw, but are stronger than Allen himself.
Allen appears in Metal Slug Attack as a playable unit in multiple incarnations; standard, enraged, and using the Rebel Gigant.
He also appears in Another Story in multiple stories.
In Metal Slug Attack, Allen is in command of "Allen Platoon", the most violent division in the Rebel Army who often sees the most wounded after an operation due to their love for fighting. Allen has several officers under his command; the only known officer in Allen Platoon is Destrade.
His robot counterpart also appears numerous times, primarily manning the Toschka Dalanue and a few other machines.
In this game, Allen first appears in a cutscene where he leads an army of Di-Cokka's to attack a Regular Army base in the middle of a desert. When Tarma, Fio, Eri, and several Regular Army soldiers try to fight off these invaders, they eventually get defeated as the Di-Cokka's destroyed many Slug tanks and killed Fio and Eri by blasting them to bits. Later on, in the underground ruins, Marco confronts Allen trying to blow up the ruins with a gigantic drilling machine but luckily he was able to disable it from exploding. Allen then orders his men to blow up the ruins with Marco in it with dynamite barrels, but all of his men get annihilated by a revived Eri (who was disguised as one of his goons). Angered, Allen shoots a load of dynamites before retreating hoping it'll blow up Marco and Eri but the two of them were able to run out of the way in time.
At the enemy base, Morden calls in Allen to eliminate Marco, Tarma, Eri and Fio before the four could apprehand him. Allen fights them, only to be defeated once again.
His design may have been based on the 80s action hero John Rambo (played by Sylvester Stallone), due to him being half-naked and armed with a heavy machine gun similar to the character in Rambo: First Blood Part II.
"YAHAHAHAHA!" -Allen O'Neil
"Come on, Boy!" -Allen O'Neil
"Go home to mommy!" -Allen O'Neil
"You're mincemeat!" -Allen O'Neil
"See you in hell...!" -Allen O'Neil
Hints that you're going to fight against Allen is by paying attention to the map geometry. If you encounter a large room, with four platforms in each corner, it is a sure sign. Allen navigates the fight by hopping between platforms, executing three of his abilities, depending on distance.
His main attack is using his M60, shooting off quick, silent bursts, so watch out. He can also aim it upwards, which is very deadly if you're on the upper platforms.
His secondary attack is throwing fast and flashing Pipe bombs (the same shape as the ones you use) when you're out of range, accompanied by a randomly saying "Come on, boy!", "You're mincemeat!", or "Go home to mommy!".
Get too close and he'll try to stab you. Try to come up from behind him and he'll just turn around to stab you.
After he is killed, he will fall to the ground and mutter his famous phrase.
- Allen is the only character in the entire series to have spoken dialogue, while all other characters only have laughs and screams including a catchphrase.
- His son Allen Jr. borrows some of his lines in Metal Slug Advance.
- Allen is playable character and the protagonist of the games Metal Slug: Allen's Battle Chronicles and Metal Slug Mobile 3 (Remake).
- Based on Metal Slug Defense description of Cyborg Allen unit, the Cyborg Allen that appear in Metal Slug 4 is the real Allen who has retired after enduring serious injury.
- On Metal Slug Attack, SNK rename it to "Mecha Allen" and change the description completely for "lore friendly" reasons, because of this change in Metal Slug Attack, "Mecha Allen" is Described as a Robot that based on Allen O Neil data. with improved resistance
- His Haircut and Beard may resemble towards Russian Leader of Leningrad, Vladimir Lenin.
- His gun is not actually an M60, instead it's an M240 Bravo.