Answer:
It was Onesimus.
In fact, that is what the book of Philemon is all about. Paul writes a letter pleading Philemon (a friend of Pauls) to give Onesimus mercy as he returns to service after running away. It appears Onesimus accepted Christ and wanted to go back and be a good witness even though it meant going back into slavery, which was legal and widespread at that time.
Pastor Brad