Marek was the King of Ichor and the leader of Midknight, before being killed during the War of the Black.
Marek would be considered a "Second Son", as his father, Kato Youkai, was one of the First Men, and the first King of Ares. Marek detested his father from a long age, and trained for his entire young life to be able to overthrow him. Ares law dictates that if a citizen is able to defeat the King in a fair fight, they may take control of the Kingdom, but Marek failed, and ran from his Kingdom of Birth. He began travelling around Olympus, desperately searching for a way to defeat his father. Along the way, Marek became a Dead Apostle and attained many other new abilities as well. He started to expand his goals, wanting to eventually rule all of Olympus.
During his travels, he stumbled across the land of Ichor, which was practically constantly in ruin with no form of government at all. Marek quickly took control of the large island, the people being tired of the constant violence welcomed him eagerly. Marek decided he would use Ichor as his launching pad, and began strategizing. Marek decided that he would start with his home of Ares, and began manipulating a man by the name of Boukun Tokage'ou. The man was powerful and ambitious, so Marek easily manipulated both him and the current leader of the Uchiha Clan into attempting a takeover of the throne. Marek's father was incredibly old by this point, but still would have been able to defeat Boukun had Marek not interferred. Kato managed to survive, but was forced away from his Kingdom. The only witnesses were supporters of Boukun, so the fact that he had not won fairly was never made clear to the people of Ares, and he was accepted as their new King, though he was simply a figurehead for Marek.
Later, Marek formed the group Midknight, comprised of many powerful natives of Ichor. Marek also eventually found out of his father's survival, and decided to go deal with him. Marek was finally able to defeat his father Kato, but only because Kato was forced to protect his student, Karasu, who Marek decided wasn't worth killing. Some time later, during the War of the Black, Marek caught word that Boukun's son, Gaius, had challenged him for the throne, and Marek quickly made his way to the site of the battle to ensure that Boukun would win. However, before he could get there, he was intercepted by the Basilis siblings, Karasu, Taka, and Kaelan. Marek killed Karasu in the battle, entirely draining his blood, but was defeated by the remaining siblings and killed.