Horologium is a Celestial Spirit who forms a contract with the celestial spirit mage Lucy Heartfilia, using a Silver Key. In the human world, Horologium appears not as a humanoid figure, but as an animate, roughly human-sized, brown wooden grandfather clock. This clock form features long, thin black arms and legs, a glass-paned central case that houses a pendulum, and a smaller clock face set into its upper body displaying Roman numerals. The most expressive part of Horologium is the small face at the very top of the clock, which has narrow, slit-like eyes and a thin, curled mustache above a small mouth.

Horologium’s primary function is defense and protection, aligning with his dignified and caring personality. He does not possess offensive combat abilities, instead using his hardwood body to shield individuals from harm, including extreme environments like freezing snow, thick mud, or even immersion in lava. A unique feature of his protective case is that it is soundproof; voices from inside cannot be heard outside. Consequently, Horologium serves as a narrator, repeating everything the person or people within him say, typically ending his statements with a phrase like he or she says. As a Celestial Spirit, Horologium is effectively immortal in the human world provided he is not forcibly kept there past his time limit. His presence is always temporary, as after a certain period he is required to return to the Celestial Spirit World, which can leave his occupant suddenly vulnerable.

His motivation is rooted in a profound sense of duty and care for his owner and her friends. While he rarely expresses his own thoughts, his actions demonstrate a deep protectiveness. He has a notable habit of forcing his own summoning through an ability called Automatic Danger Response Mode, which activates when he senses a severe and imminent threat to a person he cares about, even without being called by his key. For example, he automatically summoned himself to shield Wendy Marvell from a powerful spell. This self-summoning ability first manifested when he protected Lucy from being absorbed by the Anima in Edolas, which he explained was possible because her growing magical power allowed him to cross through the gate on his own volition.

Throughout his appearances, Horologium plays a recurring supporting role, providing safe transport or emergency shelter. One of his most dramatic rescues occurred during the battle against the Garou Knights, where he used his immense durability to submerge himself in lava and save Arcadios from certain death. Another significant demonstration of his defensive power came when he protected Lucy, Natsu, and Happy from the assassin Jacob Lessio's powerful Transport magic. Despite his loyalty, his abilities are not without flaw or limitation. On his first introduction, his time limit expired just as he was needed, leaving Lucy defenseless against a Vulcan. Furthermore, after a major self-summoning, he requires a considerable amount of time before he can use the ability again.

Horologium’s primary relationship is with his owner, Lucy Heartfilia, whom he serves with unwavering loyalty. He also shares a connection with her other spirits, having once joined Lucy in pleading their case to the Celestial Spirit King. His interactions with Lucy's guild mates are generally limited to his role as a protective container, but he has been notably protective of Wendy Marvell and Arcadios. His development is subtle but clear: he evolves from a simple summoned tool for crossing terrain to an entity capable of independent, proactive action to save lives, demonstrating a growing bond and a keen awareness of danger that exists beyond a simple master-servant relationship.