Ken Katayanagi is one half of the Katayanagi Twins, alongside his brother Kyle. Together, they are the fifth and sixth evil exes of Ramona Flowers, with Ken being designated as the sixth ex-boyfriend. The twins are of Japanese descent and are identical in appearance, though Ken is often distinguished by his darker clothing. Their history with Ramona is complicated; she dated both of them simultaneously without either brother knowing about the other. Upon discovering this deception, they harbored no hostility toward each other but instead placed the blame entirely on Ramona. From that moment forward, they swore to always do everything together, forging an unbreakable bond that defines their every action.

In terms of personality, Ken shares his brother's defining trait of incredible arrogance. The twins are highly cunning and strategic, ranking among the most calculating members of the League of Evil Exes. Rather than confronting Scott Pilgrim directly at first, they prefer to test his abilities by sending a series of robots to attack him. They are also skilled at psychological warfare, taking time to learn about Scott's past, including his history with Kim Pine, whom they identify as a constant presence in his life. This knowledge leads them to specifically target and abduct Kim in order to draw Scott out. Additionally, they shrewdly deduce that Scott's relationship with Ramona is unstable and use this information to demoralize him during their final confrontation. While they seek revenge on Ramona for her betrayal, they also seem to understand her nature as someone who runs from relationships, showing a level of detached regard for her despite their vendetta.

Motivationally, the twins are driven by the humiliation of having been deceived by Ramona. Their shared trauma led them to vow eternal cooperation, and joining the League of Evil Exes provides them with the opportunity to exact vengeance. Their primary goal is to defeat Scott Pilgrim, not necessarily out of personal animosity toward him, but as a means of striking back at Ramona and proving that they are no longer victims of her manipulations.

Ken's role in the story is as a major obstacle for Scott to overcome. He and his brother serve as the final bosses of the fifth volume of the graphic novel series. The twins engage Scott in a protracted conflict that includes four distinct battles. The first three encounters involve Scott defeating progressively larger robots that the twins deploy at social gatherings, including a Day of the Dead party and a concert at Sneaky Dee's. After these robotic proxies fail, the twins personally abduct Kim Pine and hold her hostage in a cage, forcing Scott to face them directly. In the final hand-to-hand combat, the twins initially have the upper hand thanks to their coordinated tag-team tactics, but Scott ultimately defeats them simultaneously after being spurred on by a false message of encouragement from Ramona.

Key relationships define Ken's character. His bond with his twin brother Kyle is the most significant, as they function as a single unit in thought and action. They complete each other's sentences and have developed specialized tandem fighting techniques known as the Twin-Link. Their relationship with Ramona is one of shared betrayal, and while they show no hostility toward each other, their former romantic connection to her fuels their antagonism. In the film adaptation, the twins are notably silent, acting under the direction of Gideon Graves, and their battle takes the form of a musical duel rather than a physical fight. The anime adaptation, Scott Pilgrim Takes Off, presents alternative versions of the twins, including future selves who create a time-travel portal, and they are shown attending events with their robotic companion, Robot-01.

Regarding character development, the twins show minimal change throughout the narrative arc. Their defining characteristics are established from their first appearance as cunning, arrogant, and inseparable. Their defeat at Scott's hands serves as a lesson, and in most adaptations they respawn or return home having learned from the experience, but their core personality remains largely static.

Notable abilities separate the twins into two categories. Individually, Ken does not possess many special powers. However, he is a master of robotics, having built several sophisticated fighting machines to attack Scott, including the recurring Robot-01 and larger mechs. Together with his brother, Ken is a highly skilled martial artist. Through their Twin-Link, they execute coordinated moves such as the Double Hurricane Kick and Simul-Punch, techniques that pay homage to video game series like Double Dragon and Street Fighter. In the film adaptation, their abilities take a musical form; as dual keyboard players, they can generate destructive sound waves and summon two giant snow dragons from their speakers to attack their opponents. Their strategic mind and ability to manipulate emotions are also significant assets, making them dangerous adversaries beyond their physical capabilities.