Spartan Blood, Viking Iron

A kid named Eren, who reveres the legacy of the Spartans, is isekai'd into a Viking world. His training and Spartan fighting style prove useless against Vikings with tougher, more open fighting styles. A red-haired Viking girl named Eyvah defeats and claims him as her slave, forcing Eren to learn Viking ways while tending to her and enduring her teasing. Eren trains to survive the harsh environment. When invaders attack, he proposes a plan based on Spartan knowledge that helps push them back, then saves the Chieftain, earning his freedom and a place among the tribe. He reinvents his fighting style around a short spear and short shield for greater maneuverability and accuracy. Eyvah and Eren train and fight together, gradually becoming fond of each other. More battles follow, and the next generation of Vikings begins leaving its mark on the clan.