Yū Katsuragi

One of the heroines of the story. Kei's childhood friend from long ago. It was also her father who taught Kei how to play mahjong. She harbors romantic feelings for Kei and worries that he might be involved in something dangerous. Desperate to have the old, cautious Kei back, she teams up with Ueno to try to defeat Kei in a gambling mahjong match, but Kei sees through it and she fails. As a result, she and Kei become estranged for a time.

Later, she's half-deceived by Ueno into facing Kei as a substitute player and hostage for the Shoushinkai. During the match, she's put in a situation so extreme that "even a yakuza man would scream," but she realizes that becoming a valuable hostage would only cause problems for Kei. Without seeking help from Kei, she continues to play mahjong and confesses her feelings at the table. When Kei wins the match, Shoushinkai proposes to buy the freedom of Ueno and herself, but she refuses and says a final goodbye to Kei. Ultimately, they are saved by Takatsu, who buys them for 50 million.

In the "Sacrifice Arc," she meets Kei again. In what should have been a fatal situation, Kei takes her place for the sake of ultimate victory and challenges Hatori as Kei's substitute player. Angered by Hatori’s attitude, she stabs and kills him. Afterwards, she becomes even more devoted to Kei, portrayed as somewhat unbalanced.

Prompted by Seki, she participates in Russian roulette mahjong with the aim of killing Takatsu to free Kei. However, she is defeated and comes last. She fires the live round at Takatsu but fails, resulting in being shot by Honda and falls unconscious.

In the "Massacre Arc," she recovers and returns to her normal life. However, she loses her memory of the events from the "Sacrifice Arc" due to her unconscious state. In the epilogue, she marries Kei. Her resemblance to Kei has been noted by readers, which was originally because there was an idea of "Yu pretending to be K to play mahjong when K cannot" at the start of serialization.