Who stopped the plague at Peor by killing an Israelite man and a Midianite woman?

Answer: Phinehas

Phinehas son of Eleazar, the son of Aaron, drove a spear through both of them, and the plague against Israel was halted.

“On seeing this, Phinehas son of Eleazar, the son of Aaron the priest, got up from the assembly, took a spear in his hand, followed the Israelite into the tent, and drove the spear through both of them—through the Israelite and on through the belly of the woman. So the plague against the Israelites was halted,”— Numbers 25:7-8 (BSB)