Gen is a young boy who appears as the central character in the episode Thread of Light. He is a child of considerable physical stature, described as being nearly twice as large and strong as other children his age. This natural strength, however, is a source of social difficulty, as he struggles to control his anger and is perceived as a bully by his peers, regardless of his intentions.

Gen lives with his father in a village, and his life is overshadowed by the absence of his mother, who left when he was a baby. Other children cruelly speculate that she fled because she was afraid of him, a rumor his father denies while simultaneously refusing to provide the real reason for her departure. This lack of understanding contributes to his isolation and inner turmoil. Adding to his troubles, Gen has begun to see mushi, the primordial life-forms invisible to most people. This ability frightens his father, who sternly instructs him not to tell anyone about what he sees.

Despite his difficult exterior, Gen is a lonely child who forms an emotional connection with a jelly-like white mushi he observes drifting over his village. His curiosity and imagination are also evident when he discovers a supernaturally glowing kimono with no seams hidden in a room of his house. Fascinated, he wonders if it belongs to an angel, showing a more tender and wistful side to his personality.

Gen's role in the story is to be the focal point of a predicament that Ginko, the traveling mushishi, realizes is partly his own doing from an encounter many years prior. The source of Gen's unusual strength and his connection to the mushi are linked to a phenomenon involving the ether of life, the essence that connects human souls to the River of Light. A woman had once woven this glowing, living ether into a seamless garment, and Gen's situation is a direct consequence of that event. Ginko finds that his usual methods for dealing with mushi are ineffective in Gen's case, as the forces at play are more deeply tied to human emotions like love and family bonds.

The key relationship in Gen's story is with his absent mother, whose love for him is the thematic core of the episode. Her actions, taken years ago out of a powerful maternal love, are what inadvertently caused his current condition. His relationship with his father is strained by the secrecy surrounding his mother's disappearance and his father's fear of the mushi world, which only serves to isolate Gen further. Development for Gen comes not from a typical resolution, but from understanding the powerful, positive emotion that led to his mother's actions, reframing his own origin from one of potential fear and abandonment to one of profound, if tragic, love. His notable ability, beyond his physical strength, is the capacity to see mushi, a talent that places him in a dangerous position between the human world and the world of these supernatural beings.