"See you in hell..." - Allen's famous "last" words.
Allen is infamously known as the "Immortal Devil Sergeant", as even though he is constantly killed every game, he somehow manages to survive with little injury to himself.
Allen approaches the battlefield in a hot-headed and insulting manner when it comes to fighting the soldiers of the Regular Army. Allen loves fighting the enemy, and it is why he prefers to remain as a sergeant because it is the highest rank possible that allows him to fight in the front lines on a daily basis.
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.
The only thing that keeps Allen alive, besides his muscles and big-head, is his devotion and will to return home to his wife and son. Players can battle his son, Allen Jr. as an optional boss in Metal Slug Advance.
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 single-handedly 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. He always has two Rebel soldiers to help him in the fight. Defeated, he falls off the cliff and is promptly eaten by an Orca.
Allen is the first mid-boss of Mission 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.
Allen appears in the second boss fight, controlling the Toschka Dalanue. 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 appears in Metal Slug Attack as a playable unit in multiple incarnations, including the robotic variant used by the Amadeus Syndicate.
He also appears in the Another Story mode in multiple stories.
Allen is the commander of Allen Platoon, a group that loves fighting and is known to have the most casualties during operations. He was also selected by Morden to serve as the Rebel Army's supporter, training the soldiers from the ground-up. He had convinced Morden to start the supporter program despite Abigail's reservations with it.
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 apprehend 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
In Metal Slug, Metal Slug 2/X, and Metal Slug 4, Allen fights in an arena composed of four platforms: two pairs are on each side with the top platforms being longer than the bottom two. Allen navigates the fight by hopping between platforms or running at the center floor, executing four actions, depending on distance:
- His main attack is firing his M240 in quick, silent bursts. He can also aim it diagonally upwards, which is very deadly if on the upper platforms.
- His secondary attack is throwing fast and flashing Pipe bombs when the player is out of range, accompanied by a random quote.
- His melee attack is slicing the player when close in distance.
- He'll move around to avoid continuous damage. When the player or their projectiles are near him, he'll attempt to back away.
After he is killed, he will fall to the ground, drop his M240, and utter his famous phrase. In Metal Slug 2/X, Metal Slug 3, and Metal Slug 4, the dropped M240 can be picked up and acts as a Heavy Machine Gun.
Mecha Allen plays similar to the real Allen except he gets more aggressive in his second phase. His melee attack does not kill the player; instead, the player is knocked a great distance and loses their weapons.
- Allen is the only character in the entire main series to have spoken dialogue, while all other characters only laugh and/or scream.
- 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 its remake, Metal Slug Mobile 3.
- His haircut and beard resembles the Russian Leader of Leningrad, Vladimir Lenin.
- Even though Allen does not appear in Metal Slug 6, the fight against the Controller bears similarities both in the arena design and the enemy actions by distance.
- The player can manipulate his actions and even lock him in place by firing rounds at the ground in front of him.