Horologium, also known as the Clock, is a Celestial Spirit contracted to Lucy Heartfilia, holding the rank of a Silver Key. His physical form is that of a tall, brown grandfather clock. He possesses long, black arms and legs, a glass case at his center, and a small face near his top with slit eyes and a thin, twirly mustache-like mouth. The glass compartment within his body is his most defining feature, designed to protect whoever is inside. This enclosure is soundproof, preventing those outside from hearing the occupant. As a result, Horologium acts as an intermediary, repeating everything the person inside says and appending a phrase like "...he/she/they say/s" to their statements.

Regarding his personality, little is known of his own individual voice, as his role primarily involves relaying the words of others. However, his actions consistently demonstrate a deep and genuine care for his owner and even her friends. This is most evident in his habit of forcefully summoning himself to shield individuals from grave danger, a trait that defines a significant portion of his role in the story.

Horologium's primary function and motivation are rooted in defense and protection, not combat. He serves as a mobile shelter and transport for Lucy, protecting her from environmental hazards such as snow and difficult terrain. His defensive capabilities extend to combat situations, where his durable wooden body can shield an occupant from significant threats, including being submerged in lava and surviving with only minor damage. He also possesses minor healing abilities, which he claims can cure ailments like bug bites, chapped skin, and even lack of oxygen.

A key element of Horologium's role is his ability to sometimes act independently of his master's will. When Lucy's magical power grew stronger, he was able to pass through the Celestial Spirit Gate on his own to protect her. This evolved into what is called his Automatic Danger Response Mode, where he will self-summon upon detecting a severe and immediate threat. He used this ability to save Wendy Marvell from a powerful attack, and it is implied he also used it to save Lucy from the Anima in Edolas. This ability is not without limits; after using it, he requires time to recharge before he can do so again. He is also subject to a standard time limit while in the human world, after which he is forcibly returned to the Celestial Spirit World, leaving his occupant behind regardless of the circumstances.

Throughout the series, his relationships are primarily defined through his actions. His loyalty to Lucy Heartfilia is absolute, acting as her guardian. He notably saved Wendy Marvell from Hades during the battle on Tenrou Island, and he also came to the aid of Arcadios, shielding him within a pool of lava. As a Celestial Spirit, he is also immortal, unable to die unless he is somehow forced to remain permanently in the human world. Over the course of the story, his role develops from a simple tool for transport and shelter into an autonomous guardian with a powerful instinct to protect, showcasing a reliability that far exceeds his modest appearance suggests.