Multi-Mind Mayhem: Isekai Tensei Soudouki

異世界転生騒動記

Bard Cornelius, son of a noble house in the kingdom of Mauricia, carries three souls in one body: his own, the miserly Sengoku-era warlord Oka Sanai, and Masaharu Oka, a modern otaku high-schooler. Rather than one consciousness overwriting the others, all three coexist and trade control, pooling their knowledge of commerce and warfare as Bard grows up and enters knight school.