A seasoned member of the Astral Express crew and the former sovereign of Anti-Entropy who has saved Earth from annihilation. He inherited the name of the world "Welt".
Given that his official description explicitly mentions him being the former Sovereign of Anti-Entropy and "the incident with St. Fountain" depicted in A Post-Honkai Odyssey, as well as his official character CG showing Frederica Nikola Tesla, Void Archives, and Welt Joyce, it is clear that he is supposed to be the same Welt Yang as in Honkai Impact 3rd. A developer interview confirms this.
The second Herrscher of Reason from his world, having found himself in the Star Rail world-cluster after the events of the Alien Space manga. His main weapon is a copy of the 9th Divine Key Star of Eden that he replicates using his Reason powers, taking the form of a cane.
The wise and sophisticated former Anti-Entropy Sovereign who inherits the name of the world — Welt. He has saved Earth from annihilation time and time again.
After the calamity ended, the heavy burden ordained by fate to Welt had been removed for a time, and he became an animation storyboard artist. However, after the conspiracy with St. Fountain came to a close, Welt had no choice but to venture with the initiator of the incident to the other side of the space portal.
Perhaps even he didn't expect the new journey and companions that awaited him.