Mimir, otherwise known as the Whisperer or the Nameless, is a God who was beheaded by the Vanir
At the end of the war between the Æsir and the Vanir, the two groups exchanged citizens as a sign of trust. Mimir was sent by Odin to live with the Vanir, but the Vanir found him to be a spy. So they beheaded him. Odin brought the head to life, and, with new powers it told him prophecies.
Mimir also watched over the well of knowledge. When Odin came seeking this knowledge, Mimir made him pay an eye for it.
In Runemarks, Mimir was trapped in a stone head, in a pit in World Below. Maddy and Loki free him, and he quickly causes trouble. He aspires to get an actual body through Maddy so he can rebuild Asgard, but ends up in Adam Scattergood's body instead. In Runelight, he still possesses Adam Scattergood, but he wants to use Maggie, who manages to fight back.