A.T. Robertson Commentary 2 Corinthians 8:22

"and we have sent with them our brother, whom we have many times proved earnest in many things, but now much more earnest, by reason of the great confidence which [he hath] in you." — 2 Corinthians 8:22 (ASV)

Our brother (τον αδελφον ημων). Not Paul's personal brother, but a brother in Christ, one whom Paul had tested and was willing to trust. It may have been Tychicus or Apollos, but we do not know.