In his human form he appears as a tall man with long, straight black hair that covers over his right eye. His hair is tied into a small ponytail behind his back.
He has red eyes with small rounded spectacles and usually wears an old fashioned suit (Makoto compared him to a butler when they met).
In his full dragon form, Fafnir has a black serpentine body and tentacle-like wings, six red eyes, and a vertical mouth lined with large fangs. He is also capable of assuming a monstrous purple-skinned humanoid form.
According to Tohru, Fafnir is a cursed dragon who stayed in a cave with a piles of treasure and put curses on whoever try to steal his treasure. He is greedy, stern, aloof, and initially hated humans until he met Takiya Makoto, a co-worker of Kobayashi's. Takiya introduced Fafnir to video games, which he became addicted to, and permitted Fafnir to stay at his apartment. Due to spending time with Takiya, Fafnir has became more accepting of humans, though he maintains an aloof attitude, causing Takiya to joke that he is a tsundere.