Magus is a Human Sorcerer-Apothecary who originates from the Log Horizon: West Wind Brigade side manga. In it, Magus serves as the main antagonist of the first arc, who instigates the Player Killing in Akiba and Hamelin's enslavement of weaker Adventurers.
While initially presented as a male character, Magus turns out to be a girl in real life trapped in a male avatar. After her defeat and redemption, she uses an Appearance Reset Potion to change back to a female body. She occasionally reappears throughout the manga as a side character.
Magus' original avatar bore slight resemblance to Shiroe, but had blank, psychotic eyes, especially prior to Magus' change of heart. He had pointy, unruly, auburn-colored hair and wore a long collared jacket over his clothes.
After taking an Full Appearance Reset Potion, Magus changed to a female body. Her hair is now significantly longer, and she dons a Gothic-lolita styled dress underneath her old jacket.
Magus appears to be a sadistic twisted person who loves to see other players suffer and die by Player Killing. He seems to be enjoying murder despite not doing the dirty work, although the plans he formulates are rather successful. He instigated PKing because he desired to create a world were people would fight: against other players, dangerous monsters, and pitting blade against blade in battle.
He adamantly continued to view the world as a game, which is why he wanted to make things "like they were before".
Log Horizon: West Wind Brigade
Magus first appears when Isami confronts two Hamelin guild members, who are trying to force Touya and Minori into joining. Under the guise of trying to help the two newer players, he successfully deflects Isami's suspicion and recruits the two children. However, after Isami leaves, it turns out that he incited the two Hamelin members to attempt to rape Isami and Sara. Although it fails, the subsequent fight involving Soujiro and a Royal Guard causes the first death in Akiba and reveals the immortality of the players, resulting in the drastic rise of Player Killing.
During Canossa's retreat after encountering the West Wind Brigade, Magus, Passhita, and Coza meet up with them in the forest. Magus asks them whether the PK was going well, and sounds delighted to hear about what the guilds have been doing. Canossa then gives Passhita some of the loot as one member asks Magus why they were PKing to receive EXP Pots rather than giving him gold. Magus simply responds that he needed an instigator, much to the confusion of the others. When the West Wind Brigade realizes about the rise of PK, Soujiro, Nazuna, and Isami end up confronting Magus, where Soujiro attempts to take down Magus only to discover that Minori and Touya were being held hostage. Unable to do anything, Soujiro lets Magus go, apologizing to Minori for being unable to save them. Afterwards, Magus goes alone to a cave, delivering EXP Pots to Isaac.
After being defeated by West Wind Brigade and amending his ways, he obtained an Appearance Reset Potion through unknown means and changed himself into a girl to match her real life gender. She now sports long, unkempt hair and a long black dress.
She makes a reappearance to intervene in a conflict between the Crescent Moon Refreshment Stand and the Earthlings in order to levitate rising tensions due to large demand for the secret to making real quality tasty food.