|Game||Tales of Vesperia|
|Occupation||Member of Siren's Fang|
|Japanese Voice Actor||Tadashi Tsujin|
Seifer, along with Patty, is part of the Siren's Fang (海精の牙 Kaisei no Kiba / "Seiren no Kiba"?, "Fang of the Sea's Essence") pirate guild led by Aifread. The guild disbanded after the "Black Hope" incident, in which the crew were turned into monsters due to Alexei Dinoia's actions. Seifer survived the incident and gradually turned into a demon thereafter, but prior to his transformation, he delivered Aifread, who reverted to a child due to the incident, to land, while he himself fled to the sea to die. By the events of the story, he has fully transformed into a demon and is fought by the protagonists.
The truth behind the Black Hope incident is shrouded in myth. In the story, Patty is revealed to be Aifread at some point, but most believe Seifer to be Aifread. The incident, a direct result of Alexei's experiments, forced Seifer to kill the crew, spurring the rumor that Aifread slaughtered the members of his own guild in a drastic betrayal. This same experiment caused Patty to revert to childhood and develop amnesia. At the beginning of the story, she is searching for a treasure she believes Aifread hid in an attempt to regain her memories.
Patty eventually recovers her memories in the Abysmal Hollow, where Seifer buried the guild members of Siren's Fang. She later finds the Maris Stella (麗しの星 Uruwashi no Hoshi / "Marisu Sutera"?) in Zaude and uses it at Capua Nor to call the Atherum ghost ship, where the group faces the demonic Seifer, now a skeletal creature wielding an anchor-shaped weapon. After the battle, Patty decides to kill Siefer so that he may finally know peace and continues traveling with the group, now fully dedicated to their mission.