In Romans 16:13, whose mother does Paul say had been a mother to him as well?

Answer: Rufus

Paul greets "Rufus, chosen in the Lord, and his mother, who has been a mother to me as well" (KJV: "his mother and mine").

“Greet Rufus, chosen in the Lord, and his mother, who has been a mother to me as well.”— Romans 16:13 (BSB)