volunteer soldier
A soldier who serves voluntarily, often in a militia or irregular force, as opposed to a conscript or professional soldier.
彼は義勇兵として戦争に参加した。
He joined the war as a volunteer soldier.
義勇兵 is a volunteer who fights out of conviction or patriotism, whereas 傭兵 fights mainly for money.
Formed from 義勇, voluntary courage in a public cause, and 兵, “soldier.”