Buri
Buri is one of the first children of Ymir, and the grandfather father of Odin, Vili & Ve.
An alternative rendition of his birth was that he was 'licked' out of the rime by Audulma, a cow said to exist at the beginning of everything. Not many people actually believe in this however, even the Aesir and Vanir.
It is said he was the first being to have an attunement to a Rune, though which Rune this was, nobody knows.
He went into a self imposed Exile many thousands of years ago, and handed the realm of Asgard over to his son Odin. Nobody knows where he is, or if he is even alive.