Graves is a lighteyed Alethi soldier and commander. He calls himself a patriot, yet is a member of a conspiracy to assassinate King Elhokar.
We are patriots, as I said before. Patriots of Alethkar. The Alethkar that could be.
Graves introduces himself to Kaladin in the back room of a tavern, at which are present Moash, Danlan, Rill, and one other unidentified man. He indicates to Kaladin that Elhokar is incompetent, that he gets good men killed, and that this is why he - Graves - will try to assassinate the King. Further, he indicates that the conspiracy doesn't want a coup, or to kill innocent guards; they just want the King removed quietly, preferably in an accident.
Graves' intent in removing Elhokar is to provoke Dalinar to assume the throne, indicating that Alethkar would be far better off with The Blackthorn at its head.
Graves is a little older than Adolin, with jet-black Alethi hair, styled crisply. He wears an open jacket, an expensive-looking black shirt underneath that is embroidered with white vines running between the buttons, and a stock at his throat.
Graves' men confirm him as their patron when confronted by Kaladin in the palace when the assassination attempt is in play. When the assassination attempt fails, Graves encourages Moash to kill Kaladin, who has been defending the King.
"He has seen too much," Graves said to Moash. "If he lives this day, he'll betray us. You know my words are true, Moash. Kill him." 
In the battle between them, Kaladin summons Syl at her urging, which is backed by the Stormfather.
During Kaladin's struggle with Moash, and Graves, the latter spoke of the Diagram:
"The Diagram spoke of this," Graves said, scuttling back along the corridor. "We missed it. We missed it completely! We focused on making certain you were separated from Dalinar, and not on what our actions might push you to become." 
Upon fleeing the warcamps with Moash, Graves responds to the question Moash has posed about just what is coming; the Everstorm.
"The Diagram is vague," Graves said. "We only knew the term because of old Gavilar's visions. The Diagram says this will probably return the Voidbringers, though. Those have turned out to be the parshmen, it seems." He shook his head. "Damnation. That woman was right."
Given Graves' knowledge of the Diagram and his intention to bring a Shardbearer (i.e., Moash) back with him to (likely) Taravangian, it seems that he has been an agent of Taravangian all along.
Who are you really? Moash asked.
A patriot, Graves said. Just like I told you. We're allowed to pursue our own interests and goals until we're called up. He shook his head. I thought for sure my interpretation was correct, that if we removed Elhokar, Dalinar would become our ally in what is to come ... . Well, it appears I was wrong. Either that, or I was too slow.