GTRE lore dump: Vessel's origin.
Vessel originated from an AU where he was the twin brother of Undyne. Him and his sister were part of the Royal Guard with Vessel working as Undyne's body double.
One day, in the multiverse, Ink and Error had a fight where Error pull his strings to destroy a lot of AUs. Vessel's AU was destroyed because of this event. Although Core managed to save everyone in his AU, they couldn't find Vessel's code.
Vessel's code drifted across the Void before it entered an AU where he was resembled again.
He woke up at a strange place, it was a coastal city at the shores of a desert area. And in the city, he sees humans and monsters are living in harmony. Lot of the humans have colorful hair and can use magic as opposed to the Frisk from his AU, and they all look like they came from a fantasy story.
But he can't really recall what happened. As he's lost most of his memories and the only few things that he remembers were his name, and vague memories of his sister and the human.
Zanshi Youmei, who was in the city at the time, noticed Vessel being confused. Zanshi walks up to Vessel, asking him where he came from because she never seen the type of clothes that Vessel is wearing. Vessel told her that he doesn't remember anything besides his name, and after hearing his stomach growls, Zanshi brought him to a nearby tavern.
At the tavern, Zanshi noticed Vessel's physique while he indulges himself with a hearty meal. Zanshi thought to herself that Vessel might have some fighting experiences, but she can't fully believe it until she proves it for herself.
After leaving the tavern, something came at Vessel at a high speed. Vessel quickly dodged it, and it was revealed that the attack came from Zanshi herself. Vessel was mad at Zanshi, thinking that she was trying to kill him, but Zanshi quickly reveals her true intentions that she only wanted to see if Vessel is capable of fighting because she noticed his physique.
Zanshi brought Vessel to her arena at Aranseer where he excelled in the tests that she gave him. Zanshi, thinking that Vessel already proves himself, gives him one last challenge, a duel with Zanshi's daughter Jun. Jun was already spectating Vessel in his tests, and despite Jun having the advantage of her being a Chroma monster and having more combat experiences, the fight ended with a draw.
Jun and Vessel would later be promoted to Knight Captains after Zanshi retires. And although Vessel is Jun's body double, he prefers to do his own thing of seeking for lost treasures.
Basically, Error destroying the AUs in Underverse 0.4 causes Vessel to get isekai'ed to Meropos.
