Mimir

Mímir (Old Norse "The rememberer, the wise one") is the former Æsir god of knowledge and wisdom, and formally Odin's advisor before he was reincarnated as a devil at his request to Rias's peerage with one and only spare pawm piece. He's the tritagonist in Highschool DxD: Winter is Coming.

Highlt renowned for his vast knowledge on the Nine Realms, and his near-infinite wisdom, Miir was the advisor between the Æsir and the Vanir. However, due to circumstances, he was imprisoned by Odin ater Týr's betrayal in helping the frost girants escape to Jötunheim. After some centuries, he was finally free after Rias Gremory and Issei found him. At his request, he was beheaded at his request and soon reanimated and reincarnated as a devil. Currently, he's the advisor of the Devils to all mythological factions, mostly hanging on the waist of Rias or Issei. During travelling, he sets within Rias's bag.