Seviathan has dark bluish gray skin and dark green hair with gray roots. He has dull red eyes with neon green irises. He has thick eyebrows and his mouth is filled with neon green colored sharp teeth. In a portrait, he is seen wearing a green coat with neon green trimmings, green pinstriped pants and a green hat with a light green trim that has eyes and a toothed brim. His suit is accompanied by a neon green ascot and undershirt.
From what has been revealed, Seviathan used to date in Charlie during her younger years, as seen in a portrait of the two of them during what seems to be a school prom.
Helsa is Seviathan's sister. According to Faustisse, the two have a neutral sibling relationship with each other. While Helsa and Seviathan don't hate each other, they just don't appreciate each other. They are so busy with their own stuff that they don't care about the other.[note 4] And given that Helsa and Charlie share an antagonistic relationship, it is unknown if Seviathan's romance with Charlie affected his and Helsa's sibling relationship.
- Charlie being in a previous relationship was first mentioned in Vivziepop's Twitter. Pictures of Seviathan and his family can be seen with the Magne Family all over the hotel as the two families are of elite standing.
- Sevianthan's name is a reference to Leviathan, a creature that takes the form of a sea serpent in the Hebrew Bible in the Book of Job, with "S" replacing the "L".
- Before the official name was announced, fans referred to him as Harold von Eldritch.
- He is described as a "big jerk chad" kind of evil because he is ignorant.[note 5]
|Hazbin Hotel||Helluva Boss|
|Main characters||Charlie Magne • Vaggie • Angel Dust • Alastor • Husk • Niffty||Blitzo • Millie • Moxxie • Loona|
|Supporting characters||Cherri Bomb • Fat Nuggets • Sir Pentious • The Egg Bois • Razzle and Dazzle • Katie Killjoy • Tom Trench||Stolas • Cletus • Collin • Keenie • Octavia • Robo Fizz • Verosika Mayday • Wally Wackford • Striker|
|Minor Characters||Crymini • Exorcists • Helsa von Eldritch • Lilith Magne • Lucifer Magne • Mimzy • Travis||Barbie Wire • Catfish Monster • Eddie • Joe • Lin • Loo Loo • Loopty Goopty • Lyle Lipton • Martha • Mrs. Mayberry • Ralphie • Sallie May • Stella • Verosika's crew • Vortex|
|Overlords||Alastor • Rosie • Valentino • Velvet • Vox|
|Other Characters||Arackniss • Baxter • "Henroin" • Molly • Seviathan von Eldritch||Fizzarolli • Tilla • Mammon|