Who captured Kiriath-sepher and won Caleb's daughter as his wife?

Answer: Othniel

Caleb promised his daughter Acsah to whoever captured Kiriath-sepher; Othniel, son of Kenaz, took it and married her.

“And Caleb said, “To the man who strikes down Kiriath-sepher and captures it, I will give my daughter Acsah in marriage.” So Othniel son of Caleb’s brother Kenaz captured the city, and Caleb gave his daughter Acsah to him in marriage.”— Joshua 15:16-17 (BSB)