Galleom appears to be a giant, purple robot/cyborg which has simian features, and is also capable of transforming into a jet propelled tank labelled as his "Tank Form". He is covered head to toe in a heavy mechanized arsenal, possessing two rocket launchers as shoulder weapons, and powerful jets on his feet. He wears an iron mask that covers his face, as well as metallic arm braces over his bulky forearms. He is also quite agile in his simian form, capable of performing extremely large jumps, leading to the possibility that his jumps are jet propelled. In his tank form, he is much more compact, only being able to attack by either firing the missiles on his back, or simply by ramming into his foe at a high speed. As a last resort, he can set off a Subspace Bomb hidden within his head.
In Super Smash Bros. Brawl
Role in The Subspace Emissary
Galleom is the fourth boss introduced in The Subspace Emissary. Marth, Ike, and Meta Knight were the first to spot Galleom as he was traveling through the desert in his tank form. The trio quickly caught up to him, causing Galleom to transform and fight the heroes. He was defeated and then jumps off the cliff to make a tactical retreat, only for him to land onto unstable ground which gave way, and Galleom landed in the Ruined Hall, where Lucas and the Pokémon Trainer challenge him. They manage to defeat him, and, in a last desperate attempt, Galleom grabs both Lucas and the Pokémon Trainer, starts the countdown on his emergency Subspace Bomb, then jets into the sky. Lucas manages to free both himself and the Pokémon Trainer by using PK Thunder, but in the process causes them both to fall a great distance as Galleom explodes. Meta Knight manages to catch the duo just before they land, then fly them away from the Subspace Bomb's blast. Galleom is then sucked into The Great Maze, along with the trophy of Wario, and revived; he must eventually be fought again in the Maze.
Like all bosses Galleom is controllable with hacks in Boss Battles Mode. The start button will, like all bosses, self-destruct him. Its attacks are very powerful, and several of them gain OHKO power on the higher difficulties, usually Hard and above, and even on the higher difficulties in Boss Battles.
HP and damage taken
Like all bosses in Brawl, Galleom gains an additional ×0.6 damage resistance modifier when fought in co-op mode, effectively gaining 67% more health. This modifier is applied whenever the second player is in-game, and is no longer applied if the second player loses all their stocks.
Galleom has two Trophies, one featuring his Robot Form (which is obtained by using a Trophy Stand on Galleom), and the Galleom (Tank Form) trophy, which is obtained by completing Boss Battles mode on Intense difficulty. As a side note, completing Boss Battles on Intense difficulty is the only way to get this Trophy in the NTSC version of Brawl, as using a Golden Hammer on the challenge square holding the Trophy will not work (this is not the case in the PAL version, where every box can be broken).
Galleom (Tank Form)
In Super Smash Bros. Ultimate
Galleom appears in a battlefield that resembles the large hangar in Shadow Moses Island from Metal Gear Solid, where Solid Snake fights Metal Gear REX, crashing onto the ground before transforming out of his tank form and letting out a roar, jets of steam bursting from his body. Galleom appears to have most of his attack patterns from its past incarnation in Super Smash Bros. Brawl, with some new additions:
When defeated, explosions erupt across Galleom's body as pieces of it (including one of its arms) fly off before the metal monster falls over with his remaining hand on his chest, like he did in the Subspace Emissary.
The Fighters that are assigned to fight Galleom in Classic Mode at the end of their runs are R.O.B., Snake, Wolf, Dark Pit, King K. Rool and Min Min. Mega Man and Sephiroth are also assigned to fight Galleom, but not for the final round, as Mega Man fights Dr. Mario (representing Dr. Wily), then Mewtwo (upon Dr. Mario’s defeat, who represents Wily Alien) afterwards, and Sephiroth fights Galleom in the second round of his route. Usually, Boss Battle - Super Smash Bros. Brawl plays during the battle, but Guts Man Stage plays in Mega Man's Classic Mode, ARMS Grand Prix Final Battle plays in Min Min's Classic Mode, and Galeem / Dharkon plays during the boss rush.
Role in World of Light
Galleom is assigned by Galeem to guard the Base sub area of the Light Realm, which can be accessed in a jungle like area. Defeating Galleom is necessary to weaken the shield protecting Galeem.
He is later fought again during the final boss fight, along with all the others bosses except for Master and Crazy Hand. In said battle, Galleom is the first of the Light Realm bosses to appear on screen in the second phase, which could indicate him being the first boss in the game.