Gen Asagiri is a complex and pivotal character who serves as the primary strategist and negotiator for the Kingdom of Science. A young man of slim build and average height, he has a very distinctive appearance marked by bicolored hair, with the left side being short and black while the right side is white with longer strands framing his face. His eyes are dark and shadowed, giving him a perpetually cunning and tired look. Originally a professional mentalist and stage magician in the modern world before being petrified, he was revived from the stone by the powerful warrior Tsukasa Shishio, who sought to use Gen's unique skills for his own purposes.

As a self-proclaimed "mentalist," Gen is a master of psychological manipulation, a skilled liar, and an expert in reading people's intentions. He is fluent in both Japanese and English and possesses a remarkable ability to mimic voices, even being able to convincingly impersonate a woman. He also has a high tolerance for pain and a surprisingly strong will, having survived a severe injury by preparing fake blood packs hidden under his clothes. Despite his many talents, he is not a scientist; his value lies entirely in his social and strategic acumen, which he uses to negotiate, deceive enemies, and motivate allies.

Gen's personality is a study in contradictions. On the surface, he often comes across as lazy, self-absorbed, and perverted, with a penchant for speaking in playful reversed slang and a deep, unabashed love for cola. He frequently claims to only follow the side that is winning and was famously won over to Senku's cause with the promise of a single bottle of this modern beverage. However, beneath this flippant exterior lies a fundamentally good and deeply loyal individual. He is genuinely kind, as seen in his patient interactions with the young village girl Suika, and he displays moments of heartfelt sentimentality, such as when he secretly organized the entire village to build a telescope as a birthday present for Senku. While he claims self-interest as his motive, his actions consistently reveal a determination to protect his friends and a willingness to put himself in harm's way for the sake of the Kingdom of Science.

Initially sent by Tsukasa as a spy to determine if the scientist Senku Ishigami was dead, Gen's allegiance quickly shifted. After observing Senku's relentless intelligence and his plan to build a generator, Gen was so impressed that he offered to lie to Tsukasa about Senku's survival. This act of betrayal marked his official defection to the fledgling Kingdom of Science. From that point on, he became one of Senku's most trusted and vital allies, eventually earning the title of one of the "Five Wise Generals" of the kingdom, with his specific expertise being in strategy and cunning.

His role within the story is indispensable. While Senku provides the scientific knowledge, Gen provides the human understanding. He acts as the social engineer, manipulating allies and enemies alike to ensure Senku's plans can proceed smoothly. He has negotiated tense peace talks, tricked powerful warriors into submission with clever wordplay, and orchestrated complex psychological operations to sway entire armies away from their leaders. His ability to understand and predict the actions of others makes him a formidable tactician, often serving as a foil to the more straightforward and honest scientists. He has key relationships with almost every major character, from his close friendship with the craftsman Kaseki, who enjoys his company, to his mutual respect with the keen-eared listener Ukyo Saionji. His bond with Senku is particularly strong, based on a deep admiration for the scientist's vision and an unspoken but unwavering loyalty.

Over the course of the story, Gen undergoes significant personal development. He starts as a man who openly admits to serving only his own interests and being on the winning side. However, through his experiences and his growing connection to the people of the Kingdom of Science, he evolves into a selfless and courageous individual. He has survived harrowing battles, undertaken dangerous missions deep behind enemy lines, and faced death without flinching. He learns to care for others beyond his own survival, and his skills, once used for personal gain, are dedicated entirely to the collective goal of rebuilding humanity and reaching the moon. By the end of the main narrative, his unique talents have proven so essential that he becomes a diplomat, using his lifelong skills in negotiation and persuasion to forge a new future for the world.