Roselle Gustav is a Transmigrator from the modern era who arrived in 1143—more than two centuries before Klein Moretti showed up. He walks the Savant pathway and basically became a walking bridge between two worlds. The guy was prolific: he brought over tons of proverbs, literature, and technological advances that shaped the world he landed in.
Fans and scholars often credit Roselle as a major knowledge source for Klein, who relied heavily on Roselle's diary entries to make sense of his new reality. Without Roselle's records, Klein would've had a much harder time navigating the world's mysteries and power systems. In that way, Roselle's influence stretches across centuries—he was helping someone he'd never meet understand the very foundations of their shared world.
Content compiled by AnimeList.moe from publicly available sources.
