OK you said you were running it by me first so I don't mind putting my foot down a little bit.
You can put your motivations in your character profile. Saying in that profile "Sariel's reasons are unclear to everyone else" would mean no one would know IC.
It's slightly unfair for everyone else's driving emotional forces to be displayed but yours aren't.
Also, his appearance is basically Raguel with grey wings?
If he has no qualms with serving Lucifer, how did he come to this universe-changing perspective? I'm assuming he was an angel of Heaven before he became his own master? So what made him change his mind and decide the blasphemous arch-enemy of all time was actually alright, and a possible employer?
You keep talking about alchemy (without any real description on how he attained this ability, or any interactions with humans during the time in Europe that alchemy was first coming to the fore). Are you saying that this is how he creates the Serum, and not using ingredients from the Garden of Eden? Because this is quite important-it's a literal metaphor for human beings becoming more than themselves through using select pieces of Heaven and not accepting the whole. Sorry, maybe I wasn't clear on what things I needed in cement to keep this setting "mine" and not just some random angel themed battle-this is one of them.
If he serves both God and Lucifer, and then rebels against God like you say...doesn't he just work for Lucifer now, then?!
I appreciate you're trying to make a "stand alone" character, who is a master unto himself and abides not necessarily by other's rules, but my setting already has five factions that cover pretty much every base. If your character doesn't fit in with either of these five, he's likely going to water the story down?
I'm being a bit harsh now, but I'm worried this "angel with no master, no rules, and apparently no compassion to the human race" is independent of the storyline and will detract from the narrative.
"Well, notably, I think you've made apparent the motivations of other characters because they are public figures. Sariel, however, has only come to Saul, and likely said nothing to him of his motivations. For me to play this character effectively, he should remain a mystery in his entirety (except to you as the 'game master'). However, I can address your other issues as they seem to stem from a lack of common understanding for the character."
"You describe Raguel (and the other angels) with strong, beautiful features. Sariel shouldn't look like that, that was taken from him when he started serving both sides. He is pale, smaller than the other angels, almost thin, retaining none of the angel's glow and light, but a hollow, deathly palor with a ghostly emanation, rather than a heavenly light."
"To begin with, Sariel was not his own master, in fact, it is only because of recent events that he would even consider himself so. He worked for God until Lucifer's fall, and was changed into the being he is now as a necessary evil. Sariel simply saw it as another chance to serve, only now, to serve a greater purpose, to keep peace between Heaven and Hell. He served Lucifer, but was not his ally. He was a neutral messenger, someone who was capable of going to hell to retrieve a spirit, or communicate with people who God (and occasionally Lucifer) would like to speak to that might trust a spirit rather than an angel (Atheists, even Pagans, or simply people who believed in God, but not angels per se). He was also tasked with guiding certain important people to Heaven or Hell, such as Moses."
"Sariel became more and more independent, but always loyal to his task. He began to take the time he was not called on learning various things from all three worlds (Heaven, Hell, and Earth). He has become a Jack-of-All-Trades, for the most part, capable of blending knowledge in many areas to help him. He is familiar with Alchemy and Chemistry, Eastern and Western Medicine, Technology and Mechanics, but is by no means a hacker, a machinist, a chemist, an alchemist, or a doctor (even if he is a capable healer)."
"He recently decided to deny his connection to Heaven and Hell, but waited for an opening. Saul was his opening, and he took the fruit and the bark, and used his knowledge of Alchemy to mix them to create a Serum that would create a nearly angelic Human."
"Sariel does not like Michael, Raguel, or Lucifer, and has no interest in their factions, but is not independent of the rebel angels in some of his desires."