Agent Greer

Agent Greer was an agent of the Central Intelligence Agency who was secretly a mole working with Chinese spies. She was the main antagonist of Season 9.

Early Years
Steve McGarrett met Greer right after he got his trident, but before he started dating Catherine Rollins. They worked together on a mission in Marrakesh and had a sexual relationship. She suggested Steve could work for the CIA as well, so that they could work in the same city and have a real relationship.

Later, when Steve had already met Catherine but was not dating her yet, Greer encountered Catherine and warned her that Steve wouldn't commit to a relationship, but Catherine later tells Steve she didn't believe Greer.

Season 9
Greer is introduced in Cocoon when she and a her colleague Agent Miller come to Hawaii to investigate the death of another agent, Tom Hennessy, who was also a friend of Steve’s. She tells Steve that there is a mole inside the CIA, who works with the Chinese, and Five-0 conceives a plan to find them. Steve lets himself get kidnapped and tortured by the Chinese in order to feed false information to the mole, who turns out to be Greer herself.

In The Man Who Fell from the Sky, she appears with Chinese spies trying to hunt down a mole for the CIA. She gets shot by Tani Rey and is taken into custody.

Greer appears again in When the Light Goes Out, the House Is Dark, where she sells out Steve's Seal team to Omar Hassan, the son of a target they took out in Morocco back in 2002. When Steve confronts her about it, she mocks him and tells him that he can't outrun the assassins. During the gun fight in Montana with Joe, Cole, and McGarrett, Greer escapes from prison and hides from the police.

Steve tracks her down in China in Gone on the Road from which There Is No Returning, after he has arrested Omar Hassan. He confronts here and again asks her why she turned on the men and women she has served with, but she tries to shoot Steve and is shot and killed by Catherine.