Kōrin Tatsunagi is the middle child of the three Tatsunagi sisters, with Suiko as her older sister and Rekka as her younger sister. Together, they run a card shop called PSY and form the idol group Team Ultra-Rare. In the original anime continuity, she first appears as a supporting character who observes the protagonist Aichi Sendou from a distance, initially showing little interest in him. Over time, she becomes more involved, especially after Aichi falls under the influence of PSY Qualia, a dangerous psychic power tied to the card game. She works behind the scenes to help him, even gifting him a key card when he loses his way during the Asia Circuit arc. Despite her idol persona being sweet and composed, her true personality is bold, outspoken, and not easily intimidated, though she has a weakness for being pressured by others.

Starting from the Link Joker arc, Kōrin becomes a main character. Through the arrangements of Takuto Tatsunagi, she transfers to Miyaji Academy under the name Tatsunagi Kōrin and joins the newly formed Cardfight Club as its second member. There, she initially clashes with Misaki Tokura, who becomes both a rival for Aichi's attention and eventually a close friend. Kōrin is a highly skilled fighter, originally using a Royal Paladin deck before adopting other clans such as Angel Feather and later Link Joker. During the Link Joker invasion, she is Reverse-turned by the possessed Takuto but retains a degree of self-awareness. She struggles with the dilemma of protecting her cherished memories of her friends or allowing those memories to be erased if Link Joker is defeated. Ultimately, she is defeated by Aichi and loses her memories after Takuto's departure, disappearing from the lives of those who knew her.

In Legion Mate-Hen, Kōrin's existence is removed from the memories of everyone around her, just like Aichi's. She wanders the streets, having vanished from Team Ultra-Rare as well, until she senses Aichi's suffering as he struggles to contain a Link Joker Seed within the Moon Palace. This call draws her there, where the Seed temporarily restores her lost memories. With her memories intact once more, she appears as an enemy alongside the Quatre Knights, the four fighters Aichi had secretly gathered to protect his seal. She effectively acts as the leader of the group, using a Link Joker deck centered on Star-vader units and a card she calls her incarnation of sin. She faces Kai Toshiki at the entrance to the sanctuary and defeats him, and later loses to Naoki Ishida inside the Moon Palace. After the Seed is split into twelve parts and the sanctuary vanishes, she loses her memories again and leaves Aichi's side once more. In the final episode of the series, her memory loss causes her knowledge of Vanguard to regress to a beginner's level, and she no longer remembers the people from Miyaji Academy or her past fights, though she and her sisters continue their idol activities.

Her key relationships include Aichi Sendou, whom she initially observes, then supports, and later fights against and alongside; Misaki Tokura, who becomes her close friend and rival and keeps her hair ribbon as a memento; Naoki Ishida, a fellow club member she fights in the Moon Palace; and her sisters, who remain with her through the changes. Kōrin's development centers on her struggle with identity, memory, and belonging, moving from a detached observer to an invested friend and fighter who sacrifices her memories to protect others. Her notable abilities include her versatile skill with multiple Vanguard clans, particularly Royal Paladin, Angel Feather, and Link Joker, as well as her momentary resistance to the Reverse's control through sheer willpower and emotional attachment to her friends.