The Hero of the story, Kazuki is a very well-meaning guy that enjoys helping people. At the start of the story, he's caught in the crossfire between Tokiko and a Homunculous and killed, but Tokiko gives him a Kakugane as a means of resurrecting him. He uses it to create a Busou Renkin later on.
All-Loving Hero: He's nice to everybody, he tries to save everybody, and even helps people who were killing him not five minutes before, even people who were killing him five minutes previously, then stop, then betray the truce, he'll still help them!
Badass Normal: Before getting his powers, facing down a giant snake-like homunculus with nothing but a steel pipe! Guy's got some guts, even though getting them ripped out of him doesn't do him any good, he still manages to cope with it.
Hypocrite: Papillon called him this for wanting to continue fighting him despite having been killed, comparing it to how he was practically on his death bed because of his illness and wanting to fight to continue to live. It's when Kazuki fights Papillon and says that he should stop killing and accept his fate that he begins to call Kazuki this, when Kazuki got a second chance. Of course the entire thing falls flat because Kazuki doesn't have to kill people in order to survive. That chances after the whole Victor thing, but once again falls flat because Kazuki fights to find an alternative that means not having to siphon life from people.
Walking Wasteland: His Kakugane is actually the third Black Kakugane, thus giving him the power to counter Victor, though almost at the cost of his life-drain aura becoming stronger and killing everybody else.
Belated Backstory: Her origin is not revealed in the manga until the final issue, where it's revealed she's the only person in her school who survived a homunculus attack, and ended up killing the homunculus (who had posed as a fellow classmate) with the Valkyrie Skirt, using the Kakugane which was being used to heal her wounds. She gained both her scar and her hatred of homonuculi during the incident.
Boring, but Practical: Other alchemists get flashy Stock Superpowers. Tokiko just gets a bunch of extra limbs ending in sharp objects. Fortunately, it turns out that having extra limbs ending in sharp objects is incredibly good for killing people.
Averted for the most part, though, due to Watsuki making deliberate decisions to avoid this. For example, her Valkyrie Skirt was going to be an actual skirt (as in, the blades come out from under her skirt,) until he realized that it would result in a lot of accidental Panty Shots.
Tsundere: She becomes angry whenever Kazuki does something really stupid, but she truly comes to love him.
Weak, but Skilled: Compared to Kazuki, her Busou Renkin specializes in precise and swift movement, as opposed to giant energy blasts. As Kazuki himself notes, it's almost completely lacking in flashy special abilities like most of the others, but the device's range of motion and her perfect control of it is beyond anything modern science could reasonably create, and incredibly deadly in its own right.
Badass Normal->Empowered Badass Normal: In the second episode, he is riding around totally calmly on Washio's back, even when Washio is attacking and defending - from a few hundred feet up in the air. Including perfectly holding his balance during dives and blurringly-fast maneuvers. Not bad for a guy with an Incurable Cough of Death. Then he becomes a homunculus and gains his Busou Renkin...
Combat Sadomasochist: One of the saner examples of this (and more of a masochist than sadist), but he has shown what seems like arousal in reaction to injury, and on the sadist front, look at his commentary after eating his family.
Do Not Call Me Paul: Ever since he became an homunculus, he no longer responds when called "Koushaku Chouno". He only makes an exception to his Worthy Opponent Kazuki though.
Face Palm of Doom: Delivers one to his asshole brother, and keeps it rolling from there. He appears able to consume people through creepy orifices in his palms—he just sucks them right in, leaving behind nothing but their empty clothes.
Fashion-Victim Villain: Invoked to the extreme when he becomes a homunculus and dons...this◊. Most characters are utterly weirded out by him and his bizarre fashion sense.
Immortality Immorality: Hommunculi have to eat people, but he doesn't, its the desire of the homunculi to become human that drives them to consume human flesh, since he hated his human life, he doesn't have to eat them.
Offstage Villainy: Its indicated that in order to transform himself, people had to die, including children, although Chouno didn't kill them himself (his minions probably did), but after that, he doesn't really do anything evil (although he makes threats that sound like those of an Omnicidal Maniac).
Freudian Excuse / My Greatest Failure: It's doubtful he was ever nice, but his Blood Knight and General Ripper tendencies really came to the fore after the massacre in Tokiko's school, much like with Captain Bravo. Both were deeply affected by that event: Bravo became near suicidal in his desire to protect others, Hiwatari just went "Screw this, the world is insane anyway, we might as well be crazy, too".
A rookie Alchemist Warrior that was apprenticed by Tokiko. Fell in love with her when she gave him a reason to become an Alchemist Warrior. Strongly dislikes Kazuki at first, not just because of jealousy, but also because Tokiko puts herself at danger to help him. Uses Gear Chakram.
Guest Star Party Member: Only appears in the second part of the series, but is one of the main characters from then on.
Heel-Face Turn: Decides to quit hunting down Kazuki for Tokiko's sake. This does progress into an actual friendship, even if not an easy one.
Originally named Lunare Nikolaev, Moon Face is a homunculus with a moon-shaped head and a higher-up of LXE He is depicted as constantly cheerful, rarely seen without smiling, and he uses the catch phrase "Moon~" whenever possible. After LXE is defeated, Moon Face is put into custody by the Alchemy Warriors, and later sent to the Moon with Victor and the other homunculi. Moon Face joined the LXE a hundred years before the start of the series, when he came from Russia to Japan for reasons unknown.
Meaningful Name: his head looks like a moon his name is also a play on "moon phase", which refers to his Buso Renkin's ability to create a duplicate representing every phase of the moon during a month.
Around a century ago back when Bakushaku Chouno was a normal human, new sciences and technologies were flooding in from all over the world. One of them was alchemy which most people dismissed as charlatanry, however Bakushaku sensed there was more to it and so he began to delve into its mysteries. To his disappointment he found that there were limits to how far he could go on his own. His studies had consumed a fortune and years of his life went by until he discovered the homunculi and the Kakugane which led him to a dead end. However, not long after he met Victor Powered, a traitor of the alchemist warriors, the two then joined forces. Victor was severely injured so in exchange for his knowledge of alchemy, Bakushaku promised to heal him when he had finished his research.At some point later Bakushaku became a humanoid homunculus and discarded his identity, now calling himself Dr. Butterfly. He then went on to found the L.X.E.