Onesimus is very dear to Philemon on account of the fact that he is a Christian slave and thus someone who will serve him diligently (as a man). Additionally, he is a brother in the Lord, and thus someone who join and serve in the cause of the gospel.1
16 no longer as a bondservant but more than a bondservant, as a beloved brother especially to me, but how much more to you, both in the flesh and in the Lord.