Valen is one of the Aeon and the creator of DewPrism and Dolls. He's also the main antagonist of the game.
Valen values himself above others and has every reason to reshape the universe with his relic to his desires. He even neglects any feelings for his creations such as Rue or Ruecian and won't be hesistate to dispose of them.
Prior to the story, Valen was one of the Aeons, powerful magicians that were gifted godlike attributes. Valen during his life created the Dewprism, a relic which could reshape reality. Valen intended to use its powers to make himself a god but before he had achieved his plan, he perished. Before he died, however, he created several dolls, artificial life forms which he specifically instructed to find the relic and bring him back to life.
Fortunately, one of his dolls, Ruecian (Doll Master) had awoken from his slumber and wandered the world to find Valen's relic and use it to resurrect him.
Both the stories include Valen either possessing Clarie or Rue despite having no corporeal body and using them to fight against the protagonist of that route. When he is finally defeated, he uses the Dewprism to grant him another body. Unfortunately, his final plans were halted thanks to either Rue or Mint (Depending on the initial path you took before starting the game.)
At the end of the game, when both stories are completed, a secret ending is unlocked. Mint storms into Rue's house informing him that Klaus may have found another relic that might be in ancient ruines near the Western Ruenis. Ruenis (another doll that was awakened in the secret ending which now lives with Clarie and Rue) informed them that the ruines Klaus is talking about in the letter is of Ephlesia City. In that same area Valen conducted experiments which could mean they're might be more dolls out there who might try to resurrect him.