Jimsy is a ten-year-old boy who becomes Conan's first true friend of a similar age. He is introduced living a largely solitary existence on a small island, where he survives by hunting and gathering, showing a particular fondness for catching lizards and frogs. His appearance is wild and unkempt, with long, brown hair he often keeps in a ponytail, and a year-round tan. Despite being slightly shorter than Conan, he possesses extraordinary physical abilities that rival those of the protagonist, demonstrating immense strength, speed, and courage that seem impossible for his small frame. His preferred weapon is a bow, which he uses with great skill in hunting.

Initially, Jimsy is characterized by a fiercely independent and somewhat selfish personality. Having raised himself without parents, he is cynical, pragmatic, and deeply suspicious of others. He has a strong distrust of women, considering them to be cowardly and gluttonous, and displays a general disregard for social niceties. A defining trait is his voracious appetite; he is a big eater whose favorite food is frogs, and he is known to offer smoked frogs as a gesture of goodwill. Reflecting his feral upbringing, Jimsy has picked up a number of adult vices, including a fondness for smoking and drinking alcohol, habits he openly pursues despite his age. His lack of social graces often manifests as unpredictable, comical, or blunt behavior, providing much of the series' comic relief.

His introduction in the story comes when Conan arrives on his island searching for his friend Lana. The two boys initially clash as rivals, fighting and trying to outdo one another, but this conflict quickly forges a deep and lasting friendship. Jimsy's initial motivation is purely self-preservation, but he is drawn into Conan's quest after witnessing Conan’s selfless bravery, particularly when Conan takes a severe punishment on his behalf aboard the ship Barracuda. This act of sacrifice moves Jimsy, awakening a sense of loyalty that defines his role for the rest of the series. He becomes Conan's steadfast companion, joining him in his journey to the industrial city of Industria and later to the agricultural island of High Harbor.

Key to Jimsy's character is his development from a cynical loner into a loyal and dependable team member. His relationship with Conan is the primary catalyst for this change. While Conan is idealistic and driven by his promise to protect Lana, Jimsy provides a grounded, pragmatic counterpoint. Over time, his initial misogyny is softened through his friendship with the kind and resourceful Lana, and his selfishness gives way to a genuine willingness to help others. Upon arriving at High Harbor, he becomes a pig herder, forming a complicated but ultimately friendly bond with the ornery Orlo and his sister Tera. By the end of the series, Tera chooses to accompany him, signifying his acceptance into a community. In the final battle within the Giganto warplane, Jimsy teams up with Captain Dyce to sabotage the vessel from within, fighting alongside Conan until the very end. He survives the ordeal and emerges as one of the young leaders of the new colony established on Conan's home island.

In terms of notable abilities, beyond his superhuman strength and agility, Jimsy is a masterful hunter and tracker. His skills with a bow are highly effective in both combat and survival situations. He demonstrates a clever, cunning nature and a knack for mechanical mischief, at one point using power lines with Dyce to cause a critical short-circuit and explosion. While not as strong a swimmer or as gifted with lung capacity as Conan, his raw physical power and endurance are portrayed as being on par, making him a formidable and invaluable ally in a world rebuilt from catastrophe.