Kel'Thiel was a legendary Angel who led the Angel army during the War in Heaven prior to the formation of Dark Angels and Massacre of Angelus House which he was responsible for. After allying with his rival Lucifer, he helped in the founding of Dark Angels. He soon defected and became a Demon known as The Keeper, after the capture of Lucifer he was free and later killed by Tariel. He came back from the dead however, due to Paradox's refusal to let his vessel die and came back as an Arch-Angel that he once was however began experiments to fuse himself into a being stronger than a Dark Angel, he prevailed to an extent after the Third and Final War in Heaven he became complete. He has been branded as the biggest threat to the Multiverse since Omega.
He is responsible for many major events and is considered a pinacle antagonist, due to agreements with the Arch-Angels and secretly working with them on many anti-Dark Angel experiments he has the privilage of Arch Angel powers for his own self-seeking desires, such as learning every technique, spell, power and becoming the most supreme in the Multiverse. Due to Paradox he is able to live indefinatly and has freewill. He suffers from a mental condition that is caused by Paradox's signal that makes him hear drums of war calling him to war. When the Dark Angels were removed from reality he was surpisingly angry as he had a plan to become lord of the Dark Angels and absorbing them into himself, becoming infinitely powerful.
Tariel still loves his brother, we wants to save him and give him a chance to redeem his ways, however Dark Tariel hates Kel. Kel seems to hold some love for his brother but holds more jelousy and resentment that overlaps his love for him when he grew up in Heaven. When Gabriel finds out about Kel and how he massacred his family and killed his mother in mortal combat he swore Vengeance and sought Kel out, Kel however see's Gabriel as a child who is yet to grasp the bigger picture of the Universe, acting on emotions and impulse.
When Kels mother was supposidly killed and his father in self-Exile, he grew up without a father Sephireon and mother Lilith. He was looked after my Angelus the Ascended in his Childhood. He personally believed that his parents didn't love him and left him alone due to resentment. His relationship with Angelus was distant, he rather tried to prove himself in life, making himself stronger in the hope that it would make his parents come back and be proud of him but it never did. He soon became the Aspiring leader of the Angelic Armies at the youngest age anyone ever has at the age of 8 years old due to his amazing progression. However one day his mother who was now a Arch-Demon and father came back with a child in their arms which was called Tariel. Kel in shock, of this sudden development accused his mother and father of ling to him and soon became resentful of Tariel and claiming that he is the favorite child while he was just a mistake. After his mother explained that Lucifer didn't kill her but change her into an Arch-Demon and that only by falling in love with his father did she remember about what happened.
Kel accepted this grudgingly, he was also told not to tell anyone that she was hiding with Angelus because it is the safest place to keep Tariel away from Lucifer. Sephireon went on to become the Light-bringer once again to fight at the wars front lines and Angelus announced that Tariel was her own child in order to protect Lilith, Kel became Leader of the Armies of Heaven at 13, Gabriel was born around the time of Kels promotion to the Angelus. Kel would lead the armies and when he came on would train with his younger brother Tariel, but it soon became apparent that Tariel was becoming more powerful than him, his natural talent was beyond anything he saw, he did not know whether to feel proud of concerned that he will become better than him. When Gabriel turned 24, he was soon surpassed by his brother who was soon to become the next Light Bringer, a role that is given to those who are so powerful that can take the training and absorb the Light into themselves, his mother and father showed that they were incredibly proud of Tariel which made Kel extremely jelous. This soon led to the biggest event in the Multiverses History.