When Repliforce scientists proved unable to develop their ideal "ultimate" combat Reploid combining a strong fighting spirit with a pacifistic, peace-loving heart, they instead decided to create two different Reploids from these seemingly irreconcilable attributes: the strong-willed Repliforce soldier Colonel and his little sister Iris, a peaceful arbitrator who acted as liaison between Repliforce and the Maverick Hunters for a time.
Early on in her time with the Maverick Hunters, Iris faced a trial by fire with her first mission, the Erasure Incident of Laguz Island. As Navigator, she oversaw the progress of Hunters X and Zero, developing strong feelings for the latter over the course of the mission.

SPOILER: Following the crash of the Sky Lagoon, Zero found Iris injured among the wreckage before squaring off against the Maverick seemingly responsible for the incident, Eregion. An ensuing encounter with Colonel with the belief Repliforce had been occupying the area kicked off the events leading to the Great Repliforce War. Iris assisted Zero over the course of the war, all the while attempting to put a stop to the conflict between the two sides, including a more intense personal battle between Zero and her brother.

While Iris was able to stave off the showdown between the two at the Memorial Hall, the respite was only temporary. They would once again find themselves on a collision course at the Space Port, where Repliforce were mounting a departure from Earth.
Though Colonel's last words were reportedly to tell Iris that he died happily, Iris' grief was too much for her to bear. As a result, she took Colonel's essence and departed Hunter Base for Repliforce's orbital staging ground, Final Weapon, where she would meet Zero one last time.

Combining Colonel's essence with her own seemingly produced the results Repliforce's scientists could not, as Iris' form became that of a large, flight-capable armored Reploid with an exoskeleton similar to the then-current models of Ride Armor. Not only was the armor resistant to damage, but striking it led to the production of numerous floating mines that tracked Zero's location. She could also fire a powerful laser from her arm cannon.

Despite her new body, however, the flaws in Repliforce's design became apparent as Colonel's core exited the armored form, providing Iris backup with its own laser blast used in tandem but also serving as the combined form's one vulnerability, which Zero ably exploited.

Following the battle, Iris was not able to survive the injuries she had sustained. With her dying breath, she tried once more to sway Zero into leaving the battles behind and joining her in a world only for Reploids. Zero, understanding the fallacy, held to his beliefs as he mourned her passing in his arms.

While Zero has remained resolute in carrying out further missions, it appears he has been unable to let go of his attachment to Iris and remains regretful of how their time together ended.