Like the other Phantomhive servants, he has a great deal of respect, and a bit of fear, for Sebastian Michaelis . He is also exceedingly loyal to Ciel , who he views as a savior of sorts. In the anime, he also has a crush on Angela, which results in him defending her against sound accusations. Nonetheless, he still aims to act in the best interests of Ciel.
He is a child-like character, prone to breaking out into tears whenever he does something wrong. According to Sebastian, he is an idiot with a bad memory that makes Sebastian feel a "deep emotion that goes beyond anger." He is exceptionally strong, and has to be very careful when doing work that utilizes his strength, lest he break or damage something. He enjoys his work very much, loves being outside, and works hard to please Sebastian and Ciel. He also said that if it were up to him what the garden looked like, he would "make it into the shape of a robot."
Finnian has short, messy, blond hair, which he keeps clipped back with red bobby pins. He has large, green eyes, pale skin and wears a straw hat, a plain top and plaid pants. It is later revealed that the reason he wears the hat around his neck is because it hides a tattoo of "S-012," left over from the days when he was used as an experiment for testing injections. In the manga he is seen bald when first picked up by Ciel and Sebastian. He is 163cm (5'4") tall.
Little is known about Finnian's past other than that he was experimented on, and that his captors killed his friends, locked him up and kept him inside. They are responsible for the tattoo on the back of his neck and the injections are likely the cause of his super-human strength. In his confinement, he was only given a small window, through which a bird appeared to him on a frequent basis.
Slightly over a year ago, Ciel picked up Finnian and allowed him to work as a gardener, which he loves doing because he used to not be allowed to go outside. It is for this reason that Finnian is exceptionally loyal to Ciel. It's unclear precisely what Finnian means by being "picked up." In the anime, it is changed so Ciel didn't pick him up, but rather, he was recruited by Sebastian. Also, he use to have a pet bird. His super-strength ended up killing it while supposedly petting it.