Ken Joshima is a member of the Kokuyo Junior High Gang, a group led by the powerful illusionist Mukuro Rokudo. He is a childhood ally of both Mukuro and his frequent partner, Chikusa Kakimoto, having shared a traumatic past with them as experimental subjects of the Estraneo Famiglia. This background of being used as a guinea pig for horrific experiments defines the core of his character and his profound loyalty to Mukuro, who ultimately saved him from that life.

Ken is an eccentric and over-confident individual, often described as animal-like in his mannerisms and behavior. He has a habit of teasing and belittling others, and his speech is punctuated by a distinctive verbal tic, often ending his sentences with "-byon". His sharp canines contribute to a slight lisp, affecting his pronunciation of certain words. Ken frequently clashes with the calm and grounded Chikusa, as their personalities are opposites; where Ken is hot-blooded, loud, and impulsive, Chikusa is patient and quiet. Despite their constant bickering, which includes Ken mocking him as a "four-eyed kappa", they are trusted partners who work seamlessly together, especially when the safety of Mukuro or their comrade Chrome Dokuro is at stake.

Ken's most defining trait is his absolute loyalty to Mukuro. He demonstrates great admiration for Mukuro, who he sees as his savior, and is willing to follow any order, even at the cost of his own life. This devotion does not prevent him from occasionally pulling small, forbidden pranks on Mukuro, such as wearing his jacket or eating his food, which invariably results in punishment via Mukuro's Six Paths of Reincarnation. A running comedic element is Ken’s habit of comparing Mukuro’s hair to a pineapple, a joke that never ends well for him.

His relationship with Chrome Dokuro is complex. On the surface, Ken treats her harshly, constantly mocking her and calling her names. He initially saw her only as a vessel for Mukuro. However, beneath this abrasive exterior, he cares for her well-being and happiness. He is frequently seen worrying about her, blushing when she is mentioned, and trying to cheer her up by buying her sweets or snacks. Whenever Chikusa or anyone else points out his concern, he instantly denies it and becomes flustered. This behavior suggests a genuine, if deeply hidden, fondness for her, as he sees a reflection of his own abandoned and lonely childhood self in her.

In terms of role in the story, Ken is first encountered during the Kokuyo Arc as an antagonist working to further Mukuro's plan to take over the Vongola Family. He engages in several fights, notably against Takeshi Yamamoto, whom he battles at Kokuyo Land using his unique abilities. After Mukuro's initial defeat, Ken and Chikusa continue to appear as allies, assisting in protecting Chrome and supporting Mukuro's later endeavors. He remains a recurring member of the Kokuyo Gang.

Ken's notable abilities revolve around his use of Animal Channels. By inserting special cartridges modeled after animal teeth over his own, he can modify his physiology to gain the characteristics of that animal. For instance, using the Monkey Channel grants him agility, the Kong Channel gives him immense strength, the Wolf Channel provides a powerful sense of smell, and the Cheetah Channel allows for blinding speed. When he activates a channel, a small, cartoonish image of the corresponding animal appears on his cheek. In the Final Future Battle Arc, he is also briefly shown wielding a Sun-attribute ring, which aligns with his active and energetic fighting style.