Moomin, sometimes called Moomintroll, serves as the central protagonist in his family and surrounding stories. His innate curiosity and enthusiasm for exploration drive him to investigate anything novel or unusual in his environment, often leading to unpredictable situations. While consistently striving to act with good intentions and maintain positive relationships, his attempts at bravery and support occasionally result in complications or unintended troubles.

He lives in Moominvalley’s Moominhouse with his parents, Moominpappa and Moominmamma. Moominmamma provides unwavering care and stability, ensuring the home remains a sanctuary of security and acceptance. Moominpappa balances responsible fatherhood with a restless spirit shaped by his orphaned past and youthful adventures. Their household embodies a bohemian, nature-connected, and tolerant dynamic.

Snufkin, a philosophical and independent traveler who values solitude, is Moomin’s closest companion. Their profound friendship includes seasonal separations when Snufkin migrates south each winter, evoking deep sadness in Moomin and highlighting his capacity for emotional attachment. Other key relationships involve the Snork Maiden, whom he considers a romantic partner, and the mischievous Little My, who resides with the family.

A defining incident showcasing his adventurous nature occurs when Moomin discovers a magical silk hat. Driven by curiosity, he hides inside it and undergoes a startling physical transformation into a monstrous creature, causing distress among friends and family. After reverting to his original form, he and Snufkin dispose of the hat by casting it into a river, pondering the potential return of its mysterious owner.

In "Tanoshii Moomin Ikka: Moomin Tani no Suisei" (Comet in Moominland), unusual occurrences prompt a critical journey. Hearing the Muskrat’s warning about an impending comet threatening Moominvalley, Moomin ventures toward the Observatory on the Lonely Mountains with Sniff and Mee to confirm the threat and protect his home. During this expedition, they encounter Snufkin, the Hemulen, and the Snorks, navigating numerous challenges that test their resolve and cooperation.

Moomin’s character embodies themes of security, belonging, and open-hearted acceptance. The Moominhouse symbolizes a refuge where "the door is always open," reflecting his family’s ethos of inclusivity. His actions and relationships reinforce values of kindness, loyalty, and a willingness to embrace the unknown, even amid perilous or emotionally complex situations.