Who was the advisor to odin?
His uncle, the Jotun Mimir. Or ... more accurately, Mimir's
head. Mimir was sent as part of an exchange of hostages to show
faith to the Vanir tribe of Gods after their war, along with
Hoenir. Hoenir was not the brightest of people, and relied on
Mimir's council. When the Vanir saw this, they cut off Mimir's head
and sent it back to Odin. Odin then basically pickled the head and
spoke magical charms over it, and it would speak to him when he
consults it.