Irene is a formidable warrior who served as the number two ranked Claymore during the generation of Teresa, though she was later demoted to number three with the rapid ascension of the prodigy Priscilla. She is widely known by her epithet, Quicksword Irene, a title earned through her mastery of a unique and devastating sword technique of her own creation.

Standing at one hundred eighty centimeters tall, Irene possesses the conventional physical traits of a Claymore, including silver eyes and pale blonde hair, which appears silver in some interpretations. Her expression is almost always calm and unreadable, a reflection of the immense internal control required for her fighting style. As an offensive type warrior, she possesses enhanced regenerative abilities, though they are not as potent as those of defensive types.

Irene's personality is defined by an extraordinary degree of calm, calculation, and intelligence, making her an excellent strategist in the field. This outward composure, which can make her seem cold or emotionless, is not a true lack of feeling but a necessary discipline for her signature technique. The Quicksword demands absolute control over one's emotions, channeling all of one's yoki energy into a single arm and essentially causing only that limb to partially awaken while the rest of the body remains human. This dangerous balancing act requires a clear mind and unwavering focus, qualities that Irene embodies. Beneath this stoic exterior, she harbors deeper emotions, later admitting to Clare that she envied Teresa for having found someone to love.

Irene played a pivotal role in the ill-fated mission to eliminate Teresa, who had killed a group of humans. She was the de facto leader of the team, which included Noel, Sophia, and the young prodigy Priscilla, and was the only one briefed on the mission's details. During the ensuing battle, Irene was the first to attack Teresa with her Quicksword, only to have it easily blocked, leaving her shocked at the true extent of the number one warrior's power. It was Irene who explained to her comrades the chilling reason behind Teresa's title, Teresa of the Faint Smile, referencing the slight, terrifying smile she would show when her opponents released their yoki. After Teresa was killed and a wounded Priscilla awakened into a monstrous being, Irene was the first to engage her, but she instantly lost her left arm in the clash and was critically injured before losing consciousness.

Fearing death from either the Organization who would likely dispose of a now one-armed warrior or from a returning Priscilla, Irene chose to desert after regaining consciousness. She suppressed her yoki to become undetectable and lived in a secluded mountain valley for many years, abandoning her sword and living a simple, peaceful life. She took the risk of revealing her presence only when she sensed a familiar yoki signature resembling Teresa's, which led her to rescue Clare from the sadistic warrior Ophelia. After bringing the unconscious Clare to her hideout, Irene learned that the young woman had been Teresa's companion and sought revenge against Priscilla. Recognizing the futility of this quest but understanding Clare's resolve, Irene decided to teach her the Quicksword.

During their training, Irene explained that the technique is essentially the localized awakening of the arm and that its use pushes the user dangerously close to their limit each time. She observed that Clare's heart was too chaotic and filled with a raging fire, making her fundamentally unsuited for a skill that requires a tranquil mind. Concluding that Clare could never learn the technique properly herself, Irene made a profound sacrifice. She severed her remaining right arm and offered it to Clare, insisting that she would have more use for it. Irene expressed that Clare's continued survival was the only proof that Teresa ever existed. After Clare departed, the Organization's executioner, Rafaela, arrived to eliminate the deserter. While the immediate outcome was ambiguous, it was later revealed that Rafaela chose to spare Irene, allowing her to live on. Irene ultimately survived the events of the series, and when Clare returned to her cabin years later, she had mysteriously regenerated a new arm.

Irene's relationships were marked by deep respect and fear. She openly respected and feared Teresa's immense power and pride. She held a profound and visceral fear of Priscilla, a feeling that never left her even after years in hiding after witnessing the young warrior's terrifying potential. Her relationship with Clare was complex, initially one of a reluctant mentor who was moved by the younger warrior's desperation, leading to her ultimate act of self-sacrifice to ensure Clare could continue her quest.

Irene's combat abilities place her among the most skilled warriors of her generation. The Quicksword is her most famous technique, an attack so fast it is imperceptible to the naked eye, allowing her to appear completely still while unleashing a storm of slashes. Beyond this offensive power, Irene is highly proficient at sensing yoki, demonstrated by her ability to identify Teresa's essence within Clare from a great distance. Most importantly for her survival, she is a master of yoki suppression, capable of hiding her presence completely for years to live in anonymity. According to Organization data, she received top marks in Yoki, Agility, Spirit, and Leadership, though her physical strength was rated only a B, and her perception a B, highlighting her reliance on technique and mental prowess over brute force.