Everything we know about Michael so far, lore wise.
WARNING: SPOILERS FOR ALL SEASONS
➤ He's an high-ranking angel from the Celestial Realm, more specifically a Seraph. (nb: 18-2 and nb card: Luke - "I can do it too!")
➤ His love for sweets is the reason Luke took interest in baking. (swd: 5-12 and 23-5)
➤ He and Lucifer were equals in the Celestial Realm and no other angel shared the same status as them. Simeon referred to them back then as "the two great leaders of the Celestial Realm's legion of angels." (swd: 23-7) But currently Michael carries that position alone. (swd: 23-5)
➤ He was supposed to meet MC when they visited the Celestial Realm with Solomon but couldn't because a secret spring suddenly started drying and Michael had to go there to investigate the situation. (swd: 23-7)
➤ He used to be Mammon's mentor and tried his best to train the white-haired brother while in charge of him but wasn't able to figure out how to handle him and eventually went to Lucifer asking for help. And since Lucifer was somehow able to get Mammon to listen to him, he ended up taking over Michael's role and became his mentor instead. (swd: 27-19)
➤ He was originally the one in possession of the Ring of Wisdom but gave the ring to Solomon at a time when the human was feeling "lost" and Michael wanted to help him. (swd: 29-5)
➤ During MC's 2° stay in the Devildom when Diavolo was keeping secret from everyone that their powers had become a threat to the three realms, Michael sent the prince a message through Simeon, warning him that if whatever he was keeping a secret affected the Celestial Realm, angels wouldn't hesitate to intervene. (swd: 31-16 Hard Mode)
➤ He's the one who found the Ring of Light, counterpart to the Ring of Wisdom that Lucifer had lost during the Great Celestial War (swd: 37-9), and quickly noticed it was missing when Simeon stole it to give it to MC ( since it was the only other way to control their powers ), shooting the other angel a text stating that they needed to have a "good, long talk" and that he wanted to see him as soon as possible. (swd chat: M, "untitled")
➤ When MC was given the Ring of Light and fell unconscious because of it, Michael appeared to speak with them. And during their talk, the seraph told MC that he had pictured them to be truly wicked person due to being so well-liked by the brothers, but that he had been proven wrong after meeting them. (swd: 38-17)
➤ In that same conversation, Michael confessed to being jealous of MC. He told them: "I must admit that I'm jealous of you, MC. I wish I could have been more like you. Because I loved Lucifer and his brothers, and I lost them. Perhaps things could have been different..." (swd: 38-17)
➤ When asked, Satan explained to MC that Michael was the opposite of Lucifer appearance-wise but that they felt like twins at the same time. And when Mammon brought up Michael's adoration for Lucifer, Satan chimed in saying that in his opinion the reason Michael liked Lucifer so much was because he was the embodiment of everything he wanted to be but couldn't. (swd: 43-19)
➤ He would be the one having meetings with Diavolo when the prince took over the kingdom if he hadn't thrown the responsibility onto Lucifer in the last minute, making Michael in a way the stepping stone that led Lucifer to question his faith and start having doubts about God and the Celestial Realm in general. (swd: 44-15)
➤ When MC ended up in the past Celestial Realm through a dream Solomon sent them to accidentally by feeding them his food, Michael was the one to bring them back home. The angel told MC that they and the brothers would likely forget what happened in the dream but that it would still have an effect on all of them and he would keep an eye on them to make sure that said effect didn't have negative consequences in the future. (swd: 44-18)
➤ Luke told MC that he often found Michael in the hall where the brothers' portraits used to hang staring at the empty space, lost in thought. According to the small angel, Michael to this day still deeply misses the brothers and has regrets about the war. (swd: 49-15)
➤ He was the one to cast out the brothers from the Celestial Realm and send them to the Devildom. (swd: 50-7)
➤ According to Mammon, Michael and Simeon must've pulled some strings behind the scenes to get Luke to participate in the exchange program and be able to visit the Human World with Simeon because, with his low ranking, he'd never be the one chosen for those sorts of things and the Avatar of Greed is convinced that they're doing this because they want Luke to experience life outside the Celestial Realm, see that there's more out there than heaven, and stop thinking that angels are perfect beings who can do no wrong. (swd: 50-10)
➤ He's knowledgeable on constellations and likes them so much he had Raphael rebuilt a whole room so Human World constellations could be seen even from the Celestial Realm. (swd: 52-7) He also taught Mammon about them when he was still in his care, and later on taught Luke as well. (swd: 50-12)
➤ It was actually he who went into the Devildom and told the brothers that they had been forgiven by God and had to choose between staying in the demonic kingdom ( and making an enemy out of heaven ) or coming back to the Celestial Realm and leaving Satan behind. And in a conversation with Raphael, Michael said the reason for his disguise was that he was worried about what could happen if he showed up as himself. But Raphael quickly rebutted that by saying that the true reason Michael didn't go as himself was because he wouldn't know how to act around the brothers. (nb: 20-14 Hard Mode)
➤ Still disguised as Raphael before going back to the Celestial Realm, Michael told Luke that he wasn't surprised that the brothers didn't accept his offer to go back to heaven and actually knew that would be the outcome of it. He was also surprised not too long after when Luke, not knowing it wasn't Raphael who he was talking to, pointed out that Michael seemed sad and lonely whenever he was staring at the wall where the brothers' portraits used to be. Michael's exact response to the younger angel's words was: "...I didn't realize that's what people thought.” (nb: 20-A)
➤ When asked by Raphael if he planned to attend RAD in the future, Michael avoided giving an direct answer and left. (nb: 20-14 Hard Mode)
