Fefnir

Like his siblings, Sage Harpuia, Fairy Leviathan, and Hidden Phantom, his armor is based on that of his biological origin, X, with thematic alterations. Fefnir's armor is colored red, symbolizing his alignment with fire and earth elements. His overall design is derived from his namesake, Fafnir, a dwarf-turned-dragon from Norse mythology (not to mention his name in Japanese is Fafnir). Unlike his fellow siblings and X, he does not wear the familiar 'X' shirt they share in his design, instead wearing armor with large shoulder pads. The yellow spikes on his chest and helmet resemble the protruding fangs of a dragon's snout, and his arm cannons bear a distinct jaw motif. Fefnir uses his arm cannons for flame-based attacks, one of which incorporates firing a barrage of flames into the air, which then rain down from the sky, resembling the destruction of the biblical cities of Sodom and Gohmorrah, from which his weapons' names are derived.

In his Armed Phenomenon form, Fefnir's armor expands to become a large tank equipped with cannons and flamethrowers, all of which bear a distinct dragon motif, further emphasizing his mythological inspiration.

Fefnir loves battle, and he gets disappointed upon being told to step back. Like the other Guardians, he is unquestioningly loyal to "Master X", though he may not be quite as fervent in this loyalty as Phantom was. He is proud of his position in Neo Arcadia and sees it as a true peaceful utopia, an ironic position given his own love of violence, but one he holds nonetheless. Fefnir sees Zero as a rival mostly due to his skill in battle, but he envies Zero's past with X as well. Fefnir also has a sense of honor, and dislikes having to resort to drastic measures as seen in Mega Man Zero 2 as Fefnir mention that even though he didn't want to drop a bomb at the Resistance Base, he had to. He also likes to have clean fights with Zero 1-on-1 so he can enjoy the rush of fighting him, which Leviathan also coming to realize she enjoys the battles with Zero also, but is colder towards him than Fefnir.

They were created from X's DNA data in order to work for Neo Arcadia to clean the pollution caused by the Maverick Wars. At some point, Fefnir, along with his three siblings, became military generals of Neo Arcadia, responsible for the overall safely of Neo Arcadian inhabitants, both humans and Reploids. Fefnir became the leader of the Jin'en Gundan, Neo Arcadia's land forces.

After Anubis was defeated by Zero, Fefnir launched an assault on the Resistance Base, going through the treacherous desert in which his pupil was destroyed. This assault was thwarted by Zero and Fefnir was defeated. He was defeated again by Zero in Neo Arcadia's 'Area X', but rose again to defend the Inner Sanctum where Copy X resided. He was told, along with the other two Guardians (Phantom self-detonated earlier), to step down by Copy X as he appeared before Zero himself. Copy X was destroyed later.

When Elpizo launched a full frontal assault (Operation Righteous Strike) against Neo Arcadia, Fefnir battled alongside the remaining Guardians and killed the invading force. They would have killed Elpizo too, had Zero not rescued him.

Later when Elpizo tried to infiltrate the Sanctum of Yggdrasil (where the original X lay dormant), Fefnir tried to stop him in his Armed Phenomenon form, but he was defeated at last, though not destroyed.

Fefnir took part in a mission to investigate the wreckage of a crashed spaceship that contained the exiled Reploid, Omega, at the beginning of game. He was heavily damaged in the battle with the revealed Omega, and did not reappear again until the final battle with him, only being mentioned (along with Leviathan), once throughout the game, in which Harpuia remarks to Copy X Mk. II that "Fefnir and Leviathan are still undergoing repairs".

According to the Mega Man Zero Offical Complete Works artbook, Fefnir, Harpuia, and Leviathan were transformed into Cyber-elves at the end of the game, their bodies destroyed by Omega's explosion. Zero was protected from this fate by the Dark Elf, who shielded him from the blast with her body.