In Philemon 1:2, how does Paul describe Archippus?

Answer: Our fellow soldier

Paul greets Philemon, "Apphia our sister, Archippus our fellow soldier, and the church that meets at your house."

“Paul, a prisoner of Christ Jesus, and Timothy our brother, To Philemon our beloved fellow worker, to Apphia our sister, to Archippus our fellow soldier, and to the church that meets at your house:”— Philemon 1:1-2 (BSB)