In Paul's letter to Philemon, who was Onesimus?

Answer: Philemon's slave

Onesimus had been separated from his master Philemon; Paul, who became his spiritual father while in chains, sent him back.

“no longer as a slave, but better than a slave, as a beloved brother. He is especially beloved to me, but even more so to you, both in person and in the Lord.”— Philemon 1:16 (BSB)