Finnian, often called Finny by the other servants, is the youthful and energetic gardener of the Phantomhive household. Standing at 163 centimeters tall and being sixteen years old, he is the youngest member of the servant trio that includes the maid Mey-Rin and the chef Baldroy. He is characterized by his blond hair, bright blue or turquoise eyes, and a generally small stature that belies his incredible power. A defining piece of his appearance is the straw hat he always wears tied around his neck, a garment given to him by his master, Ciel Phantomhive.

Before being taken in by Ciel, Finnian was a victim of horrific human experimentation. He was held captive in a research institution where he was known only as Subject S-012, a brand he still carries on the nape of his neck, concealed by his beloved straw hat. During this time, he was frequently injected with an unknown substance that dramatically augmented his physical strength to superhuman levels, while also being forced to witness the deaths of his fellow test subjects. When the institution was ordered to destroy all evidence of its crimes, Finnian was nearly executed by the lead researcher. However, his immense power manifested, allowing him to kill his captors and escape. He was subsequently found and rescued by Ciel and the demon butler, Sebastian Michaelis, who brought him to the Phantomhive manor.

In terms of personality, Finnian is a naive, cheerful, and excitable boy who often behaves in a childish manner. He takes great joy in his work as a gardener, largely because it allows him to spend as much time outdoors as possible, a freedom he was denied during his imprisonment. Despite his enthusiasm, he is remarkably clumsy and lacks fundamental knowledge about horticulture, frequently causing massive destruction to the manor's gardens by accident, such as by using the wrong tool for weed killer or uprooting trees with his bare hands. Sebastian often refers to him as an idiot with a poor memory due to these frequent mistakes.

Finnian's primary motivation is a profound and unwavering loyalty to Ciel Phantomhive. He views the young earl as his savior who gave him a new name, a new purpose, and a new life, and as such, he is willing to risk his life to protect the Phantomhive family. While he is normally gentle and prone to emotional outbursts and tears, this loyalty shifts into a fierce, merciless resolve when facing enemies of his master. He maintains a respectful and slightly intimidated relationship with Sebastian, acknowledging the butler's superior skills while also being the frequent recipient of his sharp reprimands. He shares a close camaraderie with his fellow servants, Mey-Rin and Baldroy, with whom he works and trains to defend the manor.

Throughout the story, Finnian's development is closely tied to confronting the trauma of his past. While he initially struggles to control his strength for fear of hurting others, he gradually learns to channel it for the protection of his new family. This is especially evident during the Emerald Witch Arc, where his unique perspective proves invaluable. When Ciel is left psychologically shattered and blind, Finnian is the only person the young earl will allow to touch him, and Finnian responds with immense tenderness, gently caring for his master while reaffirming his promise to protect him no matter what. This moment highlights his growth from a broken experiment into a devoted and emotionally supportive guardian. His notable abilities are almost entirely physical, as his past experimentation granted him monstrous, superhuman strength capable of snapping thick tree trunks or lifting massive statues with ease. Despite his lack of formal combat training, this pure, raw power makes him a formidable asset in defending the Phantomhive estate.