Alvin Castlin

Alvin Castlin was Sana's rival in the military, and was always a step ahead of her. He rose quickly through the ranks until he was the Fleet Admiral at a young age.

Aegis first encountered him after returning to our Home World in Act 2. In the past, he had told Sana if she ever needed a favor, to invoke the name "Emmet" and he would know it's an absolute emergency. Sana and Haru used this to meet with him. He explained that he remembers his past lives as Emmet Hyduk, and because of bad things he's done in his past, he decided to be a good person in this life.

Alvin explains Pyrrha tried to recruit him to her plans, but he rejected her. He gives Aegis some assistance, and tells them about Binah, a Sefirot, waiting on the moon.

Later, on March 10th, 2161, he was present for the San Andreas Hive Assault. After the successful operation, Mobius members staged a coup de etat and overthrew the UEF. Alvin escaped with Aegis.

He resolved to fight with them using the name Emmet Hyduk, and declared his love of Sana.

Season 1
Emmet was the 16th Angel, Anael, and was one of the main antagonists of the season. He meddled in Aria's life several times in the past, and clashed with Blue Light several times. Each time he was killed, he would regenerate soon after.

Blue Light eventually learned about the Multiverse system and the reincarnation game. Emmet and Aria were two of the most frequent "winners," so he was a bit obsessed with her. At the end of Season 1, Blue Light had a final confrontation with Emmet and defeated him.

Guneva Classic
In the original game, Emmet Hyduk was played by the real life Emmet Hyduk, the younger brother of the game's GM, Loudon. He joined the game late in the story and appeared in the Wing Gundam, instantly making him stronger than people who had spent months playing the game. We would eventually learn this favorable treatment was to set up a twist wherein our ally Emmet was secretly the 17th Angel. He tried to get Aria to join him, but despite their friendship, she instead struck him down.

Maybe IRL Emmet's AIM username "X_17thAngel_X" was the real giveaway.