He is portrayed as an innocent, child-like character, but is dubbed an idiot by Sebastian. But his combination of tears, innocent looks, and willful ignorance of anything that doesn't fall into his cheerful worldview makes Finny probably the most successful of the servants at wheedling favors and money out of Sebastian. He is very carefree and is almost always seen with a happy smile on his face.
Finnian displays feats of superhuman strength, which he tries to suppress from fear of hurting others and risk of sabotaging his work. The sequence S-012 are tattooed to the back of his neck and are covered by his straw hat. He works as a gardener to spend as much time as possible outside because he was not able to do so before, and explains that he loves working for Ciel because he does not give him injections, kill his friends or lock him in a cage. He was experimented on as a scientific/medical test subject to increase physical strength before the events of the story until he was rescued by Ciel.
Spoiler: End of the series
He seems to have feelings for Angela because her kindness and beauty until he saw her true dominatrix personality. At the anime's conclusion, it is unknown whether he is alive or dead, although it is noted he was breathing before he collapsed.