Fugen Shinjin is a sennin, a type of immortal sage, from the Kongrong Mountain and serves as the newly appointed leader of its twelve most powerful members, known as the Juunisen. He shares the same age of 72 as his closest friend, Taikoubou. Fugen is well-known for his profoundly kind and compassionate nature, often considered the most peace-loving among all the sennin. He is characterized by a gentle demeanor and is frequently seen smiling, even in the most dire or dangerous situations. His approach to life and conflict is rooted in a philosophy of non-violence and negotiation. He firmly believes that if attempts at peaceful negotiation fail three times, then it becomes impossible to end a conflict without force. Consequently, his fighting style is primarily defensive, as he refuses to retaliate and instead focuses on protecting himself while trying to convince an opponent to stop fighting. Only after his third attempt at persuasion has failed will he not hesitate to strike down an adversary with lethal force.

Fugen’s primary motivation in the story is intrinsically linked to Taikoubou, whom he considers his best friend and sworn soul brother. He decided years ago that he would not allow Taikoubou to die, no matter the cost, and he sees his friend as the sole reason for his own existence. This devotion is demonstrated through a thoughtful gift he gave Taikoubou, a straight fishhook. As sennin are not allowed to eat meat, he gave the hook so that Taikoubou could continue his hobby of fishing merely to think, without causing the fish to suffer. Fugen serves as a crucial emotional anchor and confidant for Taikoubou, who in turn considers Fugen someone he absolutely cannot allow to perish. In his official role, Fugen is a student of a master named Mokutaku and also takes on Mokutaku as his own pupil. His actions in the story are driven by a desire to protect his friends and resolve conflicts as peacefully as possible, although he is prepared to make the ultimate sacrifice when necessary. At one point in the narrative, he attempts to use his own abilities in a sacrificial attack to defeat a powerful enemy, Bunchu, in an effort to shut down a dangerous reactor.

Fugen possesses a remarkable and unique ability derived from his deep mastery of physics and chemistry. His treasured paopei, or magical weapon, is a globe-shaped device called the Taikyokufuin. This weapon has the phenomenal power to manipulate atoms on a molecular level, allowing him to break existing chemical bonds and create new ones between atoms. In practical terms, this knowledge of the periodic table of elements enables him to use virtually any substance as a weapon. Beyond its offensive capabilities, the Taikyokufuin is also highly versatile, capable of functioning as a radar to detect other beings or objects and can be pre-programmed with instructions to operate autonomously if Fugen is ever separated from it. The products of his advanced knowledge are so effective that they have been described as "magical science". Fugen is a male character with black hair and brown eyes, and his appearance in merchandise is noted for its playful posing that reflects his kind personality, often using translucent paint to recreate his look beautifully.