Semek Estes is a Magic Stone that belongs to the Demon Clan. He was owned by Serin and is now in Xix's possession. Semek is able to cause his wielder to lose control for a short period of time and go on a killing rampage. When the wielder regains control they have to bare witness to the chaos they created. Semek is one of the few magic stones who can challenge Caladborg since his special ability is negating.
Semek in his demon form takes on the appearance of a young man in either his late teens or early twenties. He has tanned skin with markings etched throughout his body. His hair is blonde with a few orange streaks, there are two criss-crossed bobby pins to hold up his bangs on each side. His ears have an elfish shape similar to Serin's. He wears a loose tan shirt that is similar to something a desert traveler would wear, his shirt has some strands that criss-cross over his fingers.
Although Semek causes his wielders to go on killing sprees, he is rather calm in nature when he is not in his weapon form. He shows concern for Serin when she collapses; it seems he only cares for whomever is his wielder. He is rather sarcastic in nature and is easily amused by Serin's anger on him taking control over her actions. Semek loves to act coy with his opponenets, and enjoys mocking them.Semek may have some attraction to Serin, as he is depicted embracing her in some sketches.
Semek is stolen at the beginning of the story by a low level demon. Serin is forced into traveling to the human world to retrive the stone, she easily defeats the weak demon who had stolen it. The stone does not reappear until Vol. 5, he mentally talks to Serin as she is getting manhandled by Sophia's summoning golem. He becomes interested in her after learning how she wished to die after Astarote's death and demons shunning her for being the weaker sister. As Semek learns why the necromancer created Jae-Hyuk as an undead, he sees potential in her and offers to allow her to wield him. Right before the golem crushes Serin, she accepts Semek's offer and is easily powered up to destroy the Golem. Semek saves Serin a second time from a jealous Depore.
Once Caladborg begins to awaken, the scene shifts to Infinity and Hekalroth confronting Navarus about Semek's location since the demon stone is the only one powerful enough to challenge Caladborg. Navarus tells them that Serin stole it for rescuing Jae-Hyuk.
Semek gains the attention of Latique and Ren. He repossesses Serin to duel them, and taunts Latique for not being worthy of being his wielder.
In Cavalier of the Abyss, Semek's weapon form is shown and is used by a Caladbolg-possessed Serin to attack Jae Hyuk.
Some time after Xix was born, Serin passed Semek on to him. The only ones who are aware of this are Oski and Infinity.