Chamo, whose full name is Albert Chamomile, is a small, white ermine with a long, prehensile tail and the ability to speak. Despite his cute animal appearance, he is not a transformed human but rather a type of elf or fairy known as an ermine elf. He and the young mage Negi Springfield first met in the mountains of Wales when Negi was only five years old. Chamo had been caught in a trap, and his rescuer was the young boy who freed him; this event forged a lasting bond between the two.

Chamo's personality is a complex mix of vices and genuine loyalty. He is notoriously perverted, greedy, and self-serving, with a well-documented obsession with stealing girls' underwear. His criminal record for this habit includes over three thousand stolen pairs, which forced him to flee from magical law enforcement in his native Wales. To avoid arrest, he sought refuge in Japan by becoming Negi's familiar, a legal loophole that protects a wizard's partner from prosecution. When he first arrived at Mahora Academy, he used a false story about needing the undergarments to keep his sick sister warm to gain Negi's sympathy and formalize their contract. His schemes, which often involve voyeurism or tricks to facilitate magical contracts, frequently provoke violent retaliation, particularly from Asuna Kagurazaka, who serves as his most common and aggressive punisher.

Despite these glaring flaws, Chamo is a loyal and resourceful ally to Negi. He takes his role as a familiar and advisor seriously, demonstrating a deep well of knowledge about the magical world. He is cunning and can analyze dangerous situations, often coming up with countermeasures when others are panicking. This loyalty is balanced by his entrepreneurial greed; he acts as a self-appointed agent for Negi, specializing in brokering Pactio contracts between the young mage and his female classmates. For each successful contract, he claims a commission of 50,000 "Ermine Dollars," which motivates him to engineer romantic scenarios like the "Kiss Negi" game during a class trip to Kyoto. He addresses Negi with the affectionate and respectful Japanese term "Aniki," meaning "big brother," and calls the girls "Ane-san" or "big sister," despite being much older than them.

As a skilled practitioner of magic, Chamo's most vital ability is the creation of the magical circles necessary to perform a Pactio ceremony, a skill no one else in Negi's immediate circle possesses. He can also cast anti-telepathy spells and is a resourceful improviser in a fight, using a smoke lighter to ignite flash bangs in a diversionary tactic he calls the "Ermine Flash". He is also proficient with computers, often using them to order magical items online. In times of crisis, his advice is invaluable, though his judgment can sometimes be swayed by his own selfish desires.

Chamo's relationships beyond Negi are notable. He considers the boy his only acknowledged master and serves him with his whole heart. His dynamic with Asuna is one of constant conflict, as she despises his perverted antics and is quick to grab him by the head as a form of animal abuse when he misbehaves. He maintains a friendship with Chachazero, a sentient, doll-like weapon, with whom he occasionally spends time drinking. Chamo's character shows some development across different adaptations; for instance, in the retelling of the story found in the anime Negima!?, his perverted traits are somewhat toned down. In the sequel series, UQ Holder!, he reappears much later in life as an elderly, bearded man who has transformed into a humanoid form. While he has aged and his manner has become calmer, he retains his lecherous habits, such as stealing panties, and now serves as the caretaker of one of Negi's old workshops.