Sera

Sera (セラ), the main antagonist of Mega Man Legends 2, is the Mother Unit who rules over the systems on Elysium. Sera is serious and machine-like in personality, and does not understand why the Master trusted Mega Man so much, following the System strictly. She is accompanied by her Servitor Unit, Geetz.

Story
A long time before the events of Mega Man Legends 2, Mega Man Trigger fought against Sera, Yuna and others in order to destroy the System and fulfill The Master's dying wish. Frustrated with Yuna's decision to take a neutral stance in the battle, Sera traveled to Terra herself to fight against Mega Man. Being unable to bring her full power to bear outside of Elysium, Sera was unable to defeat Mega Man. They both expended all of their energy in the battle and Mega Man's original body was almost totally destroyed. After the battle, Yuna took the opportunity to seal both inside stasis fields, with Sera being imprisoned on Forbidden Island while Mega Man was "reset" and was sealed with Data on Nino Island.

As time passed, the events were remembered as an old legend about two goddess that watched the world, the sky goddess (Sera) who "guards the record of the ages" and the earth goddess (Yuna) who "guards the keys". When the sky goddess tried to borrow the keys from the earth goddess, they argued, and the sky goddess was imprisoned by the earth goddess on Forbidden Island.

In Mega Man Legends 2, Sera and Geetz were freed by Mega Man when he went to the Forbidden Island. Sera and Geetz were taken to the Sulphur-Bottom, where Geetz tells Von Bluecher that they need four keys to unlock the Mother Lode, which will allow them to have access to ancient technology and produce any amount of Refractors. As per Bluecher's request, Mega Man searches for the keys. After Mega Man obtained the last key, Sera and Geetz attack the Sulphur-Bottom and steal the keys, which are needed to open the Library in Elysium and execute the Carbon Reinitialization Program. Sera heads to Elysium while Geetz tries to take the Master's Genetic Sample from Mega Man, damaging him in battle. After being repaired, Mega Man goes to Elysium to stop Sera. In the Library on the Mother Zone, Sera says she could already have started the Carbon Reinitialization Program, but didn't because it would be beyond the System's limits. She goes on to say that she doesn't understand why the Master attempted to destroy the System, and is jealous of Mega Man since he was closest to the Master and able to understand his thoughts in a way she wasn't able to. She held off on using the program so that she could fight and defeat Mega Man, hoping that it would purge herself from her "troublesome emotions". Some time during the fight, Gatz tries to help Mega Man by invading her internal systems, but his sabotage attempts where not able to even damage her. After this, she changes to a larger form with wings.

After Mega Man defeats her and she starts to faint, Sera says the more she obeyed the System and tried to serve the Master, the sadder he seemed when he looked at her, and now, thanks to Mega Man, the Master would probably also smile to her as he did to him, as she finally understands what they wanted, at least a little. Yuna (in Matilda's body) appears and asks Sera to transfer her primary program over to Yuna's original body before her shell stopped working, saying Gatz had died because of her "gamble", and if she also dies, there would be no one to keep her company. Now that Elysium is no longer functioning, the Old System the Master's people shut down has started to reactivate, and being Sera's responsibility to monitor it, if she died and it took over Terra, the Master's sacrifice would be for nothing. Yuna convinces her to save herself in her original body body, with Yuna commenting she doesn't remember the last time Sera smiled. Sera, Yuna and Mega Man need to return to Terra, but the game ends with them trapped in Elysium, with no means to return.

Trivia

 * The word "sera" is Italian of evening. The word "serenade" comes from the word "sera" since a serenade is sung in the evening.
 * It's believed the character Serenade.EXE from the MegaMan Battle Network series was based on Sera.