Doll Master is an advisor of the East Heaven Kingdom. He works for Princess Maya alongside with his underlings, Psycho Master, Mode Master and Trap Master. His real name is Ruecian.
Doll Master is mysterious not much is known about him except that he has his own motives to achieve. He is manipulative, as we can see how he manage to fool everyone in the kingdom, and Maya so he could break the seal to the Dewprism. He'd do anything in his powers to fulfill the task he was given by his master, Valen including murder, destroying Carona to find Prima Doll and more.
Despite his intentions, Doll Master seemed to care deeply about his brother Rue, as he sacrificed himself before Valen could kill him. He also named his underlings and he said that they will each go on their separate ways after his goal will be achieved.
Prior to the story, Valen, one of the strongest Aeons, created dolls; one of them was Doll Master, aka Ruecian. The sole reason these dolls and Ruecian for their creation was so they could used Valen's Relic and resurrect him so Valen can finally recreate the universe as he dreamed. Not long after Ruecian was awakened, he was tasked with breaking the seal which would lead him to the relic. Unfortunately, the East Heaven Kingdom made things complicated due to the fact they were tasked to guard the relic from anyone who used the relic for his/her greedy reasons. Ruecian then, under the guise of Doll Master, became part of the kingdom and made into an advisor.
In the beginning of the story, Ruecian found his brother, Rue who's also a doll of Valen. Ruecian came to take him but Claire attacked Ruecian after he severely hurt Rue. Ruecian killed her and before he could capture Rue, Rue unleashed his true powers and attacked him, forcing him to retreat.
Later on, he revealed that Mint was genuinely removed from the eligibility for the throne by the council. However, he was still responsible for manipulating Maya and the others in order to obtain the Dewprism for his master Valen.