Irene Lew is a CIA agent that appears in all three games in the NES Ninja Gaiden trilogy. The ninja Ryu Hayabusa comes to her rescue several times over the course of the series, as she sometimes takes on the role of the damsel in distress. Though she does not appear in the Team Ninja iteration of Ninja Gaiden, she is mentioned in Ryu's backstory in the first Dead or Alive. She does, however become the female lead in Ninja Gaiden II under the codename Sonia. Her most recent appearance is in Ninja Gaiden 3: Razor's Edge.
Irene and Ryu first meet each other in a cutscene at the end of the first stage of the original Ninja Gaiden. Ryu, not knowing who she is, underestimates her and is shot in the back with a tranquilizer dart. She entrusts him with a statue wanted by the evil Jaquio, but is subsequently captured by him. Ryu manages to defeat Jaquio and rescues Irene at the end of the game.
Ninja Gaiden II: The Dark Sword of Chaos
In Ninja Gaiden II, Irene is captured and used as bait to lure out Ryu, this time by the villain Ashtar. After being wounded by Ashtar, Irene manages to escape with Ryu, but Jaquio reappears and abducts her for the purpose of using her as a sacrifice.
Ninja Gaiden III: The Ancient Ship of Doom
Ninja Gaiden III begins with a Ryu impostor supposedly killing Irene. However, Irene managed to survive her drop into the ocean, and she returns later in the game to assist the real Ryu.
Ninja Gaiden II (Team Ninja)
In Team Ninja's Ninja Gaiden II, under the codename Sonia, Irene returns as the female lead. While searching for Ryu, she is ambushed by the Black Spider ninja clan and captured, but is subsequently rescued by Ryu. She then tails after him for the rest of the game, providing assistance where able.
Dead or Alive: Dimensions
As in the NES Ninja Gaiden trilogy, Irene is a CIA agent as well as a friend and partner of Ryu Hayabusa. She appears during the Chronicle story mode of Dead or Alive: Dimensions as an aid to Ryu while he tracks down the missing Kasumi.