Irina Shido is a central character who originally appears as a human exorcist before being reincarnated as an angel. She is a childhood friend of the protagonist, Issei Hyoudou, having grown up alongside him in the town of驹王镇 during her early years. As a child, she was活泼淘气 and spent much of her time playing with Issei, whom she was initially mistaken for a boy due to her tomboyish appearance and short hair. She developed an early affection for him, going so far as to steal a kiss while he was sleeping, which she later claims as having taken his first kiss.

Born into a devout Christian family with her father, Touji Shido, serving as a Protestant church pastor and exorcist, Irina’s upbringing was deeply rooted in faith. Her family relocated to England when she was young due to her father’s involvement in church-related affairs, leading to her training as a教会战士 under the Protestant church. During this time, she was injected with a Holy Sword factor, granting her the rare ability to wield a genuine holy weapon, and she came into possession of Excalibur Mimic, one of the seven fragments of the legendary blade.

Irina is characterized by her cheerful, enthusiastic, and openly devout personality. She maintains a strong, almost狂热的信仰心 in the teachings of the Biblical God and later transfers this reverence to the Archangel Michael after learning of God's death. Despite being aware of this truth, she never abandons her habits of prayer and her fundamentally holy outlook. Her faith is so pronounced that she often lives in her own imaginative world, leading her to frequently declare her status as an angel—a habit that causes others to nickname her the "自称天使" (Self-Proclaimed Angel), which greatly annoys her. She can be rather lacking in common sense due to her cloistered religious upbringing, resulting in humorous situations such as spending all her mission funds on a forgery of a religious painting.

Her motivations are heavily influenced by her religious duties and her personal relationships. Initially driven by her mission as a holy sword wielder to retrieve stolen Excalibur fragments, her primary motivation gradually shifts toward reconciling her faith with her love for Issei. Despite being a pure angel, she struggles with "impure thoughts" regarding Issei, which cause her angel wings to flicker between white and black as she teeters on the edge of堕天 (falling from grace). Her desire to be with him without losing her angelic status becomes a significant personal challenge. She also highly values her identity as Issei’s "childhood friend," viewing this history as a special bond that sets her apart from the other women in his life.

In the story of High School DxD BorN, Irina returns to Japan as a representative of the Church alongside her comrade Xenovia Quarta. Upon reuniting with Issei, she is shocked to discover he has been reincarnated as a Devil, placing them on opposing sides of the biblical factions. After the three factions form an alliance, she is personally selected by Michael to become a reincarnated angel as one of his Brave Saints, taking the rank of Ace of Spades. Subsequently, she transfers to驹王学园, joining Issei’s class and becoming a member of the Occult Research Club, where she serves as the representative of Heaven. Her role often shifts between combat support and comic relief, providing a perspective that is devoutly faithful yet awkwardly adapting to the perverted and demonic environment around her.

Irina shares several key relationships that define her character. Her bond with Issei is the most central, evolving from childhood friends and adversaries due to faction differences to eventual fiancées. She is extremely protective of her history with him and becomes flustered easily by his perverted nature. She forms the "教会三人组" (Church Trio) with her best friend and former partner Xenovia Quarta and the former saint Asia Argento. Despite their different personalities—Xenovia being blunt and Asia being gentle—they live together, fight together, and support each other in their shared goal of being with Issei. Her relationship with Rias Gremory, initially strained due to their opposing natures as an angel and a high-class devil, grows into mutual friendship and respect as Irina realizes they are simply "normal girls" at heart.

Throughout her development, Irina transitions from a fierce enemy of devils to a close ally living among them. She accepts the death of her God and redefines her faith rather than abandon it. Character-wise, she moves from being merely a zealous exorcist to someone who values her personal happiness alongside her sacred duties. A significant moment in her development is her acceptance of her feelings for Issei, vowing to find a way to love him without堕天. Later in the timeline, she ascends to become a four-winged angel and eventually a high-ranking administrator of Heaven, while also fulfilling her childhood wish by marrying Issei and bearing a son named Makoto.

As a fighter, Irina is a highly skilled swordswoman who specializes in tricky, fast movements that complement Xenovia’s more straightforward, brute-force style. Her notable abilities include the manipulation of holy light, which she can form into weapons such as swords, whips, or homing projectiles. She initially wielded Excalibur Mimic, which could shapeshift into any form, such as a Japanese katana or a flexible whip, making her attacks difficult to predict. Later, she acquires the holy sword Hauteclere, which possesses the unique ability to purify those it cuts, particularly effective against evil energies. After her reincarnation as an angel, she gains the power of flight and defensive magic, as well as proficiency with a Holy Demonic Sword prototype that allows her to wield elemental attributes like fire, ice, and lightning.