Satan is a reincarnation of Lucifer's torn soul and a former human who died through his hands, then came back to life as a demon. Since then, he shares some of Lucifer's memories and later on even declared to be Lucifer's younger brother. He appears in the DoD Main series and acts as the protagonist in the Highschool spin-off. He wields Spatial Magic. 

Name: Satan
Human Name: unknown
Nickname: Sati
Race: Demon
Former Race: Human
Birthday: Oct 20
Zodiac Sign: Leo
Physical Age: 19
Actual Age: 200
Height: 1.87m or 6'1
Place of Birth: assumingly Starfell (State of Atheucia, Uscea)
Affiliation: Rauco, Starfell Academy (temporarily)
Relatives: unknown, Lucifer (soulmate/brother)
Magic: Space Magic
Hair Color: black
Eye Color: Red
Original Eye Color: unknown
Mana Level: 128.765 mp
Prominent Characteristics: scar on cheek, scar on chest
Phobias: Musophobia (Fear of rats and mice)
Status: Alive

Appearance: 

Satan is a tall dude with mid-length raven black hair which falls over his left eye. He has a clearly visible scar on his right cheek and has the typical red eyes and pointed ears every demon has. He wears a silver necklace with a pendant shaped like an abstract dragon wing. A scar marks his chest and forms a cross on the left side; he also has a third scar on his waist. Satan has a tattoo on his left upper arm showing the demon eye. Satan's main outfit consists of a sleeveless black waistcoat with red edges, often left open and untucked, exposing his bare chest, a black cloth with red edges around his waist that reaches his knees, held by a leather black belt with a red oval-shaped buckle, white knee-length wide trousers with black ribbon ties, a thick black wristband on his left wrist and black open-toed sandals. 
In Full Demon Mode, Satan usually gets rid of his jacket entirely. He will show his black and red backwards bent horns and spikes on his shoulders. The skin of his arms will turn black and form claws from his fingers. He will develop black structures covering his thorax that kind of remind of ribs. The most prominent change are the black and red dragon-like wings that grow from his back. 
In the Highschool series, Satan is seen wearing the school uniform of Starfell Academy, consisting of a white, buttoned up shirt, a gray jacket and gray pants with a black belt. To hide his red eyes, he wears dark green contact lenses. He also seems to like wearing caps or hats, showing himself with a brown ACDC Merchandise cap or a black hat with a red strap. 

Satan in Full Demon Mode

Personality:

Satan is a quirky and funny person often lighting up the mood with his jokes and puns. He is considered not to be a very serious person, rather easy-going and chill, even somewhat lazy. He likes to sleep a lot, to Lucifer's displeasure. He seems to have concentration problems when he is tired. Furthermore, Satan's level of motivation isn't very high, which often gives him the image of a sloppy idiot. He's chaotic and lacks organization, which often causes him to lose things or be late. He's also known to be very clumsy as he shatters cups and plates on a regular basis. Satan often wears a thick smile on his face due to his easy-going and friendly nature, however, he also tends to be very sarcastic. He often makes sarcastic remarks and jokes, a lot of times even to directly provoke someone. He can be kind of a bully towards people that are offensive towards his brother or his friend Kenji, or bullies themselves, openly saying how much he dislikes them. Satan has a clear obsession with cookies. He often complains about Lucifer keeping his cookie consumption on a reasonable level. Satan is sassy and loves listening to music. Being a free thinker, Satan has a carefree demeanor going with the 'no risk, no fun'-philosophy. He's also good at cheering people up and likes to hug them, but he can also get jealous quite easily, especially when it's about Lucifer. He is quite possessive of him.
Satan does have a more reasonable side when it comes to facing trouble. He stands up for Lucifer and would even take a hit for him, which he showed when he put himself between Maeril and Lucifer, being severely wounded in the process. Satan's worst horror is the death of his brother, which he perfectly demonstrated when he became desperate about Lucifer's deteriorating energy and health, breaking down in tears when Lucifer talked about dying. He worries a lot about him, even shouting into his face each and every time he does something unreasonable in his opinion. Satan refuses to take over Lucifer's role as he considers it just as terrible as lying and freaks out when those he holds dear are being hurt in front of his eyes, causing him to even become hyper aggressive, which is unusual for his overall laid-back personality. 

Past: 

Satan lived in Starfell as a human during the time Lucifer and Lilith's children terrorized the city. He was murdered by Lucifer directly that night. When Lucifer was forced to use the Angelic Core Separation Spell on himself in order to survive, he violated the purpose of the spell, which caused him to tear his soul apart by accident. The fragment of his soul then tried to find shelter in a new body, slipping into Satan's corpse, causing him to be reincarnated as a demon. Due to bearing this part of Lucifer's soul, Satan shared all of his memories and knew about his feelings. Lucifer at first refused to acknowledge that, even attacking and wounding Satan, until he realized what had happened. Not knowing who he is or where he came from, Satan then decided to take the name mankind gave to Lucifer, which was Satan, as his own name. From then on, he stood by Lucifer's side. When Kami sought punishment on Lucifer for his raid on the city and his killing rampage, she first dispatched Maeril to bring him to her. Lucifer refused and he and Satan started fighting her. Maeril managed to pull a dirty trick on Lucifer by taking his eyesight. When she was about to strike him down during his blindness, Satan moved himself between her and Lucifer, taking a lethal hit from her light sword right across his chest. As she then decided to kill him first, Lucifer pulled him back and out of her reach so he got nothing more but a vertical scratch on his chest. After Lucifer had managed to force Maeril to retreat, Satan collapsed from blood loss. Lucifer then took him back into the dimension of Rauco, using his still partially impaired time magic powers to accelerate the blood clotting process and thus heal his injury while still being blinded. Later on, Satan noticed something was off, being alarmed by an awkward feeling he got. He then took off to look for Lucifer, who had been captured by Kami. As Kami then proceeded to try and rip Lucifer's wings out, Satan intervened, freeing his brother and escaping with him. It was after this that Lucifer accepted Satan as family. 

Relationships: 

To Lucifer: Even though Satan regularly complains about Lucifer's strictness, he still loves him to death. Satan was the only one to completely understand how hurt Lucifer was, making him compassionate and caring towards him. Knowing how much pain his brother had to endure, Satan committed himself to trying and making Lucifer's life better and happier. Satan furthermore can get pretty salty when someone dares to attempt and hurt his brother, which he showed when fighting Nazeki who longed to kill Lucifer to prove he was the strongest of all demons. Satan would never let Lucifer down or alone, always trying to be there for him, in whatever case. 

To Yukiko Hana: After Satan saved her life, he and Yukiko became long-time friends. She even asked him to be her daughter's (Yuki's) "godfather" by keeping an eye out for her when she or her husband, Shiro, couldn't. Satan gave her that promise. He was utterly shocked when Yukiko and Shiro were killed in that terror attack in November four years ago, infuriated by the monstrosity it required to commit such a terrible crime. 

To Shirayuki Hana: Holding onto the promise he gave her mother 16 years back, Satan kept watching over Yuki after her death, donating her money every month. When he noticed she was about to awaken as a half demon, he sent out Yurai to try and prevent said awakening, but failed. He then had Yurai escort Yuki and Hiko to Rauco to educate them and make sure they stayed safe. 

To Hiko Inoue: Knowing Hiko had suffered a similar fate as Yuki and being friends with her parents as well, Satan applied the promise he gave Yukiko on her as well, protecting and watching over both, her and Yuki. 

To Kenji Sawano: Having gotten to know each other on a part-time job in the Ivory Gardens café when Satan collected more money to provide for the girls, they became close friends. Having met again in the Highschool series when Satan transferred to his class, he and Kenji act like old friends and hang around together a lot. Kenji proves to be a true friend to Satan even after knowing his true identity. 

Magic and Abilities: 

Satan has a hidden ability called "Sixth Sense" which warns him everytime something terrible is about to happen to his loved ones (Lucifer especially), manifesting in an awkward, uneasy feeling in his guts. However, that's not the only benefit he has from that hidden skill. Sixth Sense also activates when his other senses are getting blocked, making sure that he is still able to locate and hit his enemy purely on instinct. 
Furthermore, Satan possesses Space Magic, also commonly referred to as Spatial Magic or Stellar Magic. It's one of the six Infinity Forces that allows him to use spatial energy and space itself to attack. 

Spells: 

-Space Spheres (The user creates spheres by distortion of space itself, usually as a shield, to repel every attack. According to laws of space, magic in no matter what form, cannot exist in to separate areas of space, which means he easily can block every attack by surrounding himself with a space sphere.)

-Territory (The user can swap the places of objects within the same area of space. It's even possible for him to swap places with other people on the field, similar to teleporting.)

-Attack Teleport (The user can teleport attacks or weapons to other areas in space, even across the dimensions, presupposing the dimensions aren't closed off.)

-Starry Heaven (A light-based heavenly body attack that enables the user to shoot light beams from every star visible in the area onto his enemies. They can only use this spell at night. The clearer the sky is, the stronger this attack becomes.)

-88 Spheres of Universe (The user creates 88 colored space spheres around him and unleashes them onto their foes.)

-Giant Supernova (The user builds a miniature heavenly body above his enemy and makes it explode, creating devastating damage by the aftermath of this "event".)

-Tail of the Comet (The user surrounds themselves with golden energetic light, increasing their own speed.)

-Cosmic Energy Beam (The user gathers cosmic energy and fires it off from their hand.)

-Lunar Crescent Blade (Gathering moonlight, the user forms it into a crescent-shaped blade which he fires at their opponents. The blade looks like an actual crescent moon.)

-Space Lock (Ability to lock a person in one spot in space majorly to prevent teleporting.) 

-Eruption of the Sun (The user gathers cosmic energy to surround himself with a reddish-orange aura made of plasma. They then unleash an attack similar to a solar eruption.) 

-Black Hole Eclipse (The user creates a black hole by distorting space. That black hole absorbs anything in the area before it implodes and leaves drastic damage.) 

-Gravity Manipulation (This spell allows the user to in- or decrease gravity and use it to their advantage.)

-Descent Of The Falling Stars (This spell allows the user to collect cosmic energy and release several beams of this energy to hit a target.)

-Last Judgement (This spell can only be utilized when being combined with Temporal Magic. Making the power of time and space collide, an imbalance in the time-space continuum is generated, causing the distortion to expand so much it will trigger the cataclysmic destruction spell. The sky will glow in a reddish light before dozens of meteors will fall to the earth, exterminating everything in their impact zone.) 

Weaknesses:

-Due to being an infinity force, the only weakness Spatial Magic has is itself, since only space can interfere with space itself. 

Quotes: 

"Me pretending to listen should be enough for you, in fact. I'm not listening, but please, keep on talking. I enjoy the way your voice makes my ears bleed... and I'll yawn when I'm interested." -to Irina Asakawa

"I'll tell you something. If anyone dares saying that there's nothing more complicated than love, I'll throw my physics textbook in their face." -to Kenji Sawano 

"Chocolate is God's apology for broccoli." -to Kenji Sawano 

"While you were talking to me, I tried to figure out where the hell you got the idea I cared. And, just for the record, just because I say 'I don't care', doesn't mean I don't understand. By the way... in order to insult me, I first need to value your opinion. Nice try though." -to Irina Asakawa

"Wow, you're dumber than you look, and that's saying something! What kind of idiot tells their victim what they're about to do?" -to Blake McCartney 

"Your lips keep moving, but all I can hear is 'blah, blah, blah'..." -to Blake McCartney

"Joy is fleeting like a shooting star that crosses the night sky, ready to blink out at every moment. So does the present. The hard part of life is we have to keep living even when our world has stopped spinning and all the stars are laying at our feet." -to Yukiko Hana 

"I can't promise to fix all of your problems... but I can promise you won't be left to face them all alone." -to Lucifer 

"Admitting your weakness does not diminish your strength. It shows your courage. Never let your weakness convince you that you lack strength." -to Lucifer

Trivia: 

-Satan has the second highest level of mana from all Rauco demons. 

-Satan is one of six infinity force wielders. 

-Satan is the only known person that has been reincarnated from the dead. 

-Satan has Lucifer's Spatial Magic. 

-Satan shares Lucifer's memories from point of his "birth" to the moment his soul split. 

-The scar on Satan's waist was inflicted to him by Lucifer. 

-Satan has given two people a sealed version of his Attack Teleport spell. One being Tir'an, and the other one being Lucifer. 

-Satan is yet the only demon known to have been in Venetremoria state. 

-"Satan" originates from the hebrew language and means "adversary/accuser". 

-The pendant of Satan's necklace has a strong similarity with the Dragon Badge from the Pokémon Black and White Series. 

-Satan's flower is the orange-colored Tulip. This particular color makes the Tulip stand for truest love and understanding, which both describe the love and care Satan holds for his brother Lucifer. He has been the only one to ever truly and deeply understand what Lucifer has been through and what pain he suffered from, since he shared parts of his memory.