With Onesimus, a faithful and beloved brother, who is [one] of you. They shall make known unto you all things which [are done] here.
Note 2 at Col 4:9: Onesimus was the converted runaway slave mentioned in the book of Philemon. He became a faithful and beloved Christian brother of the Apostle Paul. He was a Colossian and accompanied Tychicus in the reporting of Paul's affairs and the delivery of this letter (see note 1 at Col 4:7).
The name Onesimus means "profitable, helpful" (Thayer's Greek-English Lexicon). Paul, in a letter to Philemon, seemed to play on these words when he said, "Who once was unprofitable to you, but now is profitable to you and to me" (Phm 11, New King James Version).
Note 3 at Col 4:9: One of Paul's purposes in writing this letter was to let the Colossians know how he and his companions were doing. Remember, Paul had never met these people (see note 5 at Col 1:4 and note 13 at Col 2:1), but he had a relationship with them through the messengers who had heard the Gospel from him and relayed it to the Colossians. Paul wanted to continue to strengthen this relationship, so through Onesimus, he kept them informed of what was happening in his life.
This illustrates the truth that communication is important to relationships. Paul was communicating with great difficulty in comparison to the modern means of communication we have today, but he made the effort, because he knew it was important. "A man that hath friends must shew himself friendly" (Pr 18:24).