Hollow (Empty in the Japanese version) is a being that doesn't fuse with anyone. He's hollow inside, hence the name. Hollow is Lady Vega's assistant, and a big help when it comes to fighting.
Hollow is actually Dr. Vega's dead lover, Altair (Hiko in the Japanese version). After his death, Vega used the Matter Waves to recreate Hiko, but unfortunately, Altair is merely a Matter Wave, and does not hold the original's memories. Thus, Vega wanted to use Le Mu's power to revive Altair, and for the time being, modified Altair's form to be better oriented for combat, creating Hollow.
After Geo's friends were scattered througout the world, Hollow finds Harp Note before Geo does, and manages to convince her to join Vega, if Geo were to be spared. Hollow also grants Rogue the power of the Mu Rejection barrier, and proceeds to attack Wilshire Hills with an army of Murians. When MegaMan shows up there, Hollow sics the newly powered Rogue on him as he makes his escape.
After MegaMan gets Harp Note back to his side, the both of them track him to the deepest part of the Bermuda Labyrinth, where the entrance to Vega's base is. After MegaMan takes down Hollow, Rogue shows up and fights him, and shortly after, Hollow takes the advantage of MegaMan's weakened state and seizes the OOPArt that Omega-Xis had swallowed.
Prior to the final battle on Mu, Hollow appears again, and is also interrupted by Rogue. He then uses the Kamikakushi to attempt to trap the three of them in an altenate dimension, but Rogue throws MegaMan out, telling him to defeat Le Mu, whilst Rogue and Hollow duke it out.
After Le Mu's defeat, Hollow abandons his fight with Rogue, and appears to ensure that Vega's plan goes unhindered, and MegaMan attacks, exposing the Altair Matter Wave. This is when Altair's backstory is revealed. When Le Mu begins to go out of control, Hollow protects Vega with his life, allowing her to avoid the blow, at the cost of his body. As Hollow is deleted, he thanks Vega for once again giving him life, and also apologizing that he could not fulfill her wish. He prays for Altair speak to Vega, and Altair eventually does so, telling Vega to live a long and happy life.
Hollow starts the battle with 6 specially designated panels. These are where he summons his viruses from, and if the panels are removed, no viruses will be summoned.
- Hollow Barrier : After a set time after moving, Hollow will generate a barrier around him. This can be pierced by Breaking attacks.
- Summon Virus : Hollow can summon a virus on one of the special panels.
- Dance Flame : A Dance Flame virus will be summoned, performing its flamethrower at the player.
- Chain Bubble : A Chain Bubble virus will be summoned, creating bubbles at the player.
- Stealth Laser : A Stealth Laser virus will be summoned, firing three Invis-piercing shots at the player.
- Squall : A Squall virus will be summoned, unleashing a wood tornado at the player. Removes Barriers and Auras.
- Magic Thunder : Hollow will summon lightning bolts that will strike down on the player's row and the row in front.
- Thunder Bazooka : Hollow will fire a lightning blast down the row. Another version of this will spread to adjacent panels.
Due to the lack of time as the anime was cancelled and had to wrap up quickly, Hollow's backstory was not explained. Also, Hollow is almost impossible to beat if he's hooked up to an energy source. If not, he doesn't have a chance. Hollow is defeated at the hands of Geo Stelar (Subaru Hoshiwawa) when he turns into Megaman (Rockman) with the help of Omega-Xis (War-Rock) along with the help of the three OOParts allowing him to become Tribe King.