Sodom (ソドム) is Luka's powerful dragon familiar. His true form is a huge dragon, but usually, he is in a chibi "energy-saving" form and later he appears in a human form sometimes. He can also appears in a wolf form.
He is very loyal towards Luka, having no problems attacking even his own brother, Gomorrah, (who happens to be Luze's familiar.) if Luka so orders. It may not seem like it, but Luka truly cares for Sodom as he is seen by Yuki watching over Sodom as he plays with a butterfly.
Sodom is presumably named after one of the two hedonistic biblical cities, the other being Gomorrah.
Sodom can be somewhat timid and is terrified of crows as they often attack him for trying to steal their shiny stuff. He's also afraid of Hotsuma, one of the Zweilt guardians.
However, Sodom has also shown himself to be quite brave when protecting people he cares about. He's willing to attack opponents much stronger than himself in order to protect Yuki and his master, Luka Crosszeria.
Sodom also seems to have a big appetite, wondering if a hot spring is a food and whether he can eat it.
Sodom lived in the "heavens" or the sky above the Niflheim, along with his brother, Gomorrah. He apparently had a very painful and sad life as his brother would "pick on him,"every day.
The Demon King, Lucifer, whom Luka still served at the time, ordered Luka to kill the two dragons as they were being, "unbearably noisy." He also said that every other detachment he sent to subdue them all failed.
Gomorrah put up a fight but was ultimately defeated by Luka's forces. Sodom, on the other hand, did not put up a fight and went along with Luka peacefully, so Luka spared him and Sodom asked to be his familiar.