He's the Phantom Stranger. He knows tons. 
The Phantom Stranger is an higher being of Earth-85.
History
The Phantom Stranger came into existence long before the Crisis and was one of the higher beings of Earth-85.
When the Crisis erupted, the Mar Novu sent Felicity Smoak in a quest through the Multiverse to search for the Book of Oa. Felicity tracked it energy to Earth-85 so she breached to that reality, backed up by the Ray and Nyssa al Ghul.
There, the trio discovered the Anti-Monitor fighting a multitude of heroes and were welcomed by the Phantom Stranger who explained them the dangerous situation. The Stranger suggested them to avoid searching for the Book as he was too dangerous for mortal beings like them.
As Felicity begged him to lead them to the Book, the Stranger replied that the Book could not resurrect her husband, Oliver, as she wished but he nevertheless decided to help her, teleporting the three of them to the home reality of the Book, Earth-12, knowing that their quest would be fundamental for the Multiverse's survival.[1]
Powers and Abilities
Powers
- Omniscience: The Phantom Stranger knew about Felicity's arrival to Earth-85.
- Teleportation: The Phantom Stranger teleported Felicity and the other heroes from his Earht to another reality.
Trivia
- After the rebirth of the Multiverse, another Phantom Stranger appeared on Earth-19. It is unknown if that iteration of the Stranger is the same of this Phantom Stranger of they are two different beings.
Related
- 1 Appearances of Phantom Stranger (Arrowverse: Earth-85)
- 1 Images featuring Phantom Stranger (Arrowverse: Earth-85)
- Quotations by or about Phantom Stranger (Arrowverse: Earth-85)
- Character Gallery: Phantom Stranger (Arrowverse: Earth-85)
Footnotes