Irina Shidou is a character who begins as a human Church exorcist before being reincarnated as an Angel serving under the Seraph Michael. She is a childhood friend of the protagonist, Issei Hyoudou, having grown up with him in the town of Kuoh during her early years. Her family was forced to move to England due to an incident involving her father's church work and a demon, where she was subsequently raised and trained as a warrior for the Protestant Church.

In terms of personality, Irina is most notably cheerful, upbeat, and sincere. She possesses a very strong and devout Christian faith, which is the cornerstone of her identity and actions. Early in her introduction, this faith made her harsh and judgmental toward those she considered enemies of her religion, such as devils, which created conflict with characters like Asia Argento. However, at her core, she is a benevolent and sociable person who is quick to make friends, and her religious fervor often manifests in comedic and eccentric ways. She has a habit of proudly declaring herself to be an angel, a childhood friend, and Japanese, leading those around her to teasingly call her a "self-proclaimed" version of these things. Irina also has a somewhat naive or lacking in common sense when it comes to daily life, and she developed a passion for making bread after purchasing a bread machine with Heaven's funds.

Irina's primary motivation is her devotion to God and Heaven, which later evolves to include her feelings for Issei. After learning that the God of the Bible is dead, she transfers her absolute belief to the current leader of Heaven, Michael. Her desire to serve Heaven is so strong that she accepts reincarnation as an angel when the opportunity arises. Simultaneously, she struggles with her growing romantic feelings for Issei, which conflict with an angel's prohibition against impure thoughts. Her journey involves trying to reconcile her faith and duty with her personal desires, including her wish to eventually marry Issei and have children with him.

Irina's role in the story is multifaceted. She is first introduced as an antagonist of sorts, sent to Japan with her partner Xenovia Quarta to retrieve stolen holy swords, which pits her against Issei and his demon associates. After the conflict is resolved, she is reincarnated as an angel and becomes an ally, transferring to Kuoh Academy as a representative of Heaven. She is a key member of the "Church Trio," alongside Xenovia and Asia, and later an official member of the Occult Research Club. She fights alongside the Gremory group in major battles against terrorist factions like the Khaos Brigade and the Hero Faction. In the later arcs, she becomes a knight on Issei's team for the Azazel Cup, a major international rating game tournament.

Her key relationships are central to her character. Her bond with Issei Hyoudou is the most significant; they were childhood friends who made a promise to defeat Santa Claus together, and she secretly gave him his first kiss when they were young. Their relationship is strained upon their reunion due to their opposing sides, but they later reconcile and grow closer. As an angel, Irina often finds herself close to "falling" due to her lecherous thoughts about Issei, and she eventually becomes his fourth bride. She shares an extremely close friendship with Xenovia Quarta, with whom she has worked alongside for years, despite their different personalities and Xenovia's eventual reincarnation as a devil. Together with Xenovia and Asia Argento, she forms the "Church Trio," a tight-knit group who share their faith and often pray together. Her relationships with other members of the Occult Research Club, like Rias Gremory, start off tense due to faction differences but eventually develop into strong friendships.

Irina undergoes significant development throughout the series. She starts as a zealous exorcist who sees all devils as enemies, but she matures to accept and befriend them, even falling in love with one. Her discovery of God's death is a major turning point, forcing her to redefine her faith, which she ultimately places in Michael. Her character arc also involves learning to navigate her duties as an angel with her personal, human desires. She becomes bolder and more proactive in her relationship with Issei, moving from a shy childhood friend to someone who confidently calls him "darling" and actively seeks a future with him. Professionally, she grows in strength and rank, starting as a two-winged angel and eventually being promoted to a four-winged Ultimate-class Angel.

Irina is a skilled swordswoman known for her speed and agility. Her abilities are largely tied to her weapons and her angelic powers. Initially, as an exorcist, she was an artificial holy sword user and wielded one of the seven fragments of Excalibur, specifically Excalibur Mimic, which could change its shape at her will. After becoming an angel, she gains the ability to create weapons from holy light, such as swords, bows, and arrows. She also uses a prototype of a Holy Demonic Sword, which she can imbue with elemental attributes like fire, ice, and lightning. Her most notable weapon is the legendary holy sword Hauteclere, which she inherits from her father. This sword is said to be wieldable only by those with pure hearts and has the ability to purify that which it cuts. As she advances in rank, her power grows correspondingly, making her a formidable combatant in the conflicts between angels, devils, and fallen angels.