Cancer is a Celestial Spirit who holds the key to the Cancer constellation, making him one of the twelve Golden Zodiac Keys. His key originally belonged to Layla Heartfilia before being passed down to her daughter, Lucy Heartfilia, who remains his current owner. As a Celestial Spirit, Cancer cannot die under normal circumstances, though remaining in the human world for too long without returning to the Celestial Spirit World would drain his life force.

In terms of appearance, Cancer dresses like a hairdresser. He has black hair styled in braided red cornrows that end in shapes resembling crab pincers. He has thick lips and is never seen without his green-lensed sunglasses. He typically wears a blue striped shirt with dark trousers and boots, and he has six small crab legs that protrude from his back. His primary tools are two large pairs of scissors with crab-claw design handles, which he wields with exceptional skill.

Cancer possesses a calm and cool personality, not being particularly emotional or hotheaded. One of his most distinctive traits is his verbal tic, as he ends many of his sentences with the suffix ebi, which means shrimp. He takes his role as a hairdresser very seriously and is always ready to provide his services whenever summoned. Interestingly, Cancer behaves differently in the presence of the Fairy Tail mage Erza Scarlight, acting in a more gentlemanly manner and ending his sentences with choki, meaning snip, a change Erza herself suggested. He once presented Erza with a bouquet of roses, indicating a mild infatuation or deep respect for her.

Cancer's primary motivation and role within the story revolve around his duty as Lucy Heartfilia's spirit. He assumes the role of her personal hairdresser, and she summons him frequently to cut her hair or change her hairstyle. Beyond aesthetic purposes, Cancer is a capable fighter in combat situations. He uses his giant scissors not only for cutting hair but also as weapons, and his blades are sharp enough to slice through enemy weapons such as swords and hammers. Giving an opponent an embarrassing or aggressive haircut often serves as his method of subduing them in battle.

His key relationships are primarily with his owner, Lucy, who summons him regularly for both practical and combative purposes. He also shares a notable dynamic with Erza, whom he treats with particular gallantry. Among the other Celestial Spirits, he counts Aquarius and the other Zodiac spirits as his companions, having originally served alongside them under Layla Heartfilia.

Over the course of the series, Cancer demonstrates some development in his capabilities. After Lucy unlocked her Second Origin, Cancer received new scissors that were longer with square tips, though he later returned to using his original pair. His abilities extend beyond mere cutting; he is remarkably dexterous and fast, capable of performing swift hand movements and cutting hair with extreme accuracy in a single instant. He can shave multiple opponents in mere seconds without giving them time to react. Cancer also possesses a surprising skill for lock-picking, able to use his scissors to open locks and even ancient labyrinth entrances. Additionally, he has a unique ability called Hair Growth: Scalp Care, which allows him to make others hair grow back quickly by washing and massaging their scalp.

There exists an alternate version of this spirit known as Eclipse Cancer, which appears during a specific anime-exclusive story arc. This version transforms into a muscular, dark-skinned man with a red afro, crab claws for arms, and a flamboyant personality centered around dancing. Unlike the original Cancer, Eclipse Cancer ends his sentences with kani, meaning crab, and speaks in broken English. This version challenges Gray Fullbuster to a dance battle and exhibits exaggerated flirtatious behavior. However, this Eclipse form is not part of the main canon storyline.