Allen Jr. is an antagonist and a boss who first appeared in Metal Slug Advance.
Allen Jr. is the son of Allen O'Neil, and much like his father he has become a soldier. Seeking revenge on the P.F. Squad for their victories against the Rebel Army, he has gathered Rebel troops and infiltrated the P.F.'s training camp located on a remote island, apparently intending to destroy the whole place.
Allen Jr. has a romantic relationship with the Rebel officer Julia, as the two share similar tattoos and are often paired up on missions.
Metal Slug Advance
Allen Jr. made his first appearance in Metal Slug Advance. Acting as a Drill Instructor, he was in fact a spy for the Rebel Army, as was later revealed during his betrayal of Regular Forces on the island. He tricks the player into falling into a trap that leads to the third mission's underground setting, and the player later fights him in the final mission if the correct path is taken. The card earned for beating him shares his namesake, but, unlike his father, his M60 Machine-gun cannot be taken when he is defeated.
Metal Slug Attack
Allen Jr. made his first appearance in the Extra Ops "Boy Memories", where he confronts Walter Ryan when he attempted to save several POWs that Allen Jr. captured. Allen Jr. sent the Cabrakan to kill Walter, but Walter critically damaged it, causing the Cabrakan to destroy the platform that Allen Jr. was on, presumably killing him. However, it is revealed that he survived, as he fell into a Sand Marine. Abigail was then sent to find him, but to no avail. It is presumed that he was captured by the Ptolemaic Army, but he managed to escape and reconnect to Rebel command.
He later challenges Walter to a rematch, but the Regular Army soldier proves to be more than capable and Allen Jr. retreats, abandoning his soldiers and the secret weapons given to him; a Keesi III+ and Formor+. Returning to base, he casually takes a shower, only for Julia to barge in and shame him into helping her rescue his soldiers and retrieve the Keesi III+ and Formor+. They are able to recapture them, and he proclaims to his men that he retreated in order to find reinforcements. They know that he is trying to hog the glory for himself, but Julia decides to help him save face by stating that he is a genius.
Later on, Allen Jr. and his father participating in an operation against the Regular Army, overwhelming them with their new weapons. They have a "contest" of sorts to see who can kill the most, with Allen Jr. using the new Rogue Giant. Perche arrives at the battlefield, and battles the duo. Allen Jr. tries his best to hold off Perche, but finds it very hard to do so despite her petite size. Allen abandons him after witnessing Perche's power, and when she rips off the arm of his Rogue Giant, he immediately ditches it and flees.
Following his encounter with Perche, Morden asks him to participate in a practice simulation with the Amadeus Syndicate, which he admits that he would have ignored if it came from anyone else. He is accompanied by both Julia and Emma, and battle their opponents - a large unmanned division led by an Amadeus-built Shoe and Karn. During the battle, he states that it isn't the same fighting machines, and soon runs out of ammo. Emma makes him more weapons to continue the fight, but before he can do so, Shoe and Karn end the simulation, as continuing the fight is pointless. Allen Jr. is content with their "surrender", taking it as a win in his books.
Allen Jr. is later given control of a new Element Giant. He played a small role in its development, requesting it to be painted in a dark-blue color and its weapons use a mix of both incendiary and freezing attacks. Once it was developed, he later took it out on a test run at the Egyptian ruins, with Abul Abbas serving as his guide. Testing the machine out disturbs the ruins greatly, prompting Cleopatra to wake up early from her slumber. She threatens to smite him for his sacrilege, but he eagerly accepts her challenge, knowing that a fight against a goddess would be legendary. Though evenly matched at first, the Element Giant gives in, prompting Allen Jr. to ditch it and run towards Abul and Shizuka, the latter using her ninjutsu to bring them to safety.
A while later, Allen Jr. learns of an assassination attempt on his father and promises to protect him alongside Julia and Beatriz. The assassin is revealed to be a young girl who fails to harm Allen. Allen Jr. tries to have assassin stay and fight, but she had already been convinced otherwise by Beatriz, who suggested she leave.
Allen Jr. made his first appearance in the story "The Pharaoh Treasure", where he's tasked with Beatriz to find the fabled Arabian Tear. They meet Scotia Amundsen during the mission and eventually secure the Arabian Tear.
In the story "The Back You Can't Reach", Allen Jr. is deployed with Beatriz to steal a powerful prototype weapon from the Regular Army. They successfully do so, but eventually fail due to Beatriz unintentionally luring the P.F. soldiers to their base and the destruction of the Cabrakan.
Allen Jr. makes a brief appearance in the story "Cross/You", where he asks Vita to bring him his medicine, and tries to dissuade her from drinking the suspicious liquids when she tries to prove her point but fails.
While Allen Jr. shares some similarities with his father, he is the polar opposite of him - Allen O'Neil's tactics are limited to running into battle with all guns blazing, but Allen Jr. is cool and calculating, as evidenced by his spying on the Regular Army. However, he has a similar fighting style to his father - when he has his M60 he will keep his distance but deliver a blow to the player with his ammunition belt if they get too close. If he takes enough damage, however, he flies into a berserker rage and slashes the player with a Bowie Knife.
In Another Story, Allen O'Neil describes his son as someone that refuses to listen to him, especially during the heat of battle.
His design is based on both Antonio Banderas and Rambo. He uses a M60 Machine-gun has Lewis MG's buttstock, while his father uses an M240 Bravo Machine-gun.