Seth the Assassin (reversely as Assassin Seth in Japan, better known by his extended nickname Unseen Assassin) is a main deuteragonist who is descendant of one of Night Blade’s assassins and currently hired privately by an Amnesia member Bloody Chaos (the best friend of Gordeau the Harvester), as well as the main rival of Hyde Kido. He is a playable character in Under Night In-Birth And its updates.
He returns in BlazBlue: Cross Tag Battle as one of the Season 2 DLC/Arcade Edition playable characters.
The progeny of the original Night Blade clansmen, who chose to stick to their traditions and remain operating in secrecy. Seth is a member of one of these families who reluctantly carries out his people's wishes by putting down Voids out in the Night as penance for his peoples' mistakes. 10 years ago when Seth was still young, he encountered Linne in a different body known as Chitose, as well as her bodyguard Waldstein and vowed to save her, but was unable to given that he lacked the abilities to truly kill her.
By the time after entering his first year at the same high school as Hyde Kido, Carmine Prime and Orie Ballardiae, Seth aimlessly fulfilled the duties to this day until he was given a chance from a mysterious benefactor who revealed himself to be Bloody Chaos of Amnesia, used a middle-man to deliver him the Eliminators, the weapons that could potentially help him keep his promise to Linne. In exchange, however, Seth takes up a new job as an assassin hunting out specific groups of In-Births on orders, while also searching for the Princess of the Night Blade to finally close the book on his promise. Chaos himself does not know where he got it from and deliver it to Seth.
- Seth’s late-inclusion in this game was a result of the fans not pleased with initial and Season 1 rosters by Arc System Works and other volunteering companies’ content practices.
- Due to his Amnesia client, Chaos is yet to be announced to appear in Cross Tag Battle, despite being the first one to have Vs./Team-Up Theme with Gordeau, as well as the other non-BlazBlue series sides like from Persona has yet to achieve proper Vs./Team-Up Themes, he and his main rival, Hyde are currently the only non-BlazBlue characters to receive this.
- For an unknown reason, the announcer, who is an English voice actress calls him in his Japanese pronunciation Seto during his team’s win announcement, unlike in Character Select. This is fixed at the final version of the release.
- Seth has color palettes of the following characters: Jack-O Valentine (Guilty Gear), Hibiki Kohaku (BlazBlue), Sho Minazuki (Persona 4 Arena Ultimax), Hyde Kido, Neopolitan (RWBY), and Sonic the Hedgehog (Sonic the Hedgehog).
- Ex-Night Blade members like Seth and Yuzuriha has a color reference based on Sega’s Sonic the Hedgehog on Color 13: Seth (titular protagonist), and Yuzuriha (Shadow the Hedgehog).