συστρατιώτηςsystratiṓtēs

a co-campaigner, i.e. (figuratively) an associate in Christian toil

Greek (New Testament) · G4961

Meanings

a fellow-soldier. Metaph., of fellowship in Christian service: Philippians 2:25, Philemon 1:2.

Same root

σύνsýn · G4862with or together (but much closer than G3326 (μετά) or G3844 (παρά)), i.e. by association, companionship, process, resemblance, possession, instrumentality, addition, etc.
στρατιώτηςstratiṓtēs · G4757a camper-out, i.e. a (common) warrior (literally or figuratively)

Where it appears

Appears 2 times in the Bible.

Philippians 2:25 Meanwhile, I thought I should send Epaphroditus back to you. He is a true brother, co-worker, and fellow soldier. And he was your messenger to help me in my need.
Philemon 1:2 and to our sister Apphia, and to our fellow soldier Archippus, and to the church that meets in your house.

Source: G. Abbott-Smith, A Manual Greek Lexicon of the New Testament (1922), public domain.