Philemon was a Christian known only from the letter the apostle Paul wrote to him. The New Testament does not mention him anywhere else.
Colossians 4:17 lists Archippus, a man of Colossae, along with Philemon in Philemon 1:2. Archippus may have been the son of Philemon. Although Paul had never visited Colossae, it is clear he knew Philemon well (Colossians 2:1). Paul called him “our beloved co-worker” (Philemon 1:1). Perhaps Philemon had worked with Paul during Paul's three-year mission in Ephesus (Acts 19:8–10; 20:31).
Paul knew he could appeal to Philemon on behalf of Onesimus, who was Philemon's runaway slave. The letter to Philemon focuses on Paul's request that Philemon accept Onesimus back, not just as a slave, but as a Christian brother.