legionary

A soldier belonging to a legion; a professional soldier of the ancient Roman army.

Suddenly, [...] he saw all this as romantic — the last legionary, his aloneness, the lost cause really lost — and instinctively he pulled in his paunch, stroked down his hair to cover the naked part of the scalp, and wiped the sweat off his cheeks.

A person who is neither a citizen nor colonial/imperial subject of the state whose military they join.

A member of a legion, such as the American Legion, or of any organization containing the term legion in its title (e.g. the French Foreign Legion).

Relating to, or consisting of, a legion or legions.

a legionary force

Containing a great number.

1646, Sir Thomas Browne, Pseudodoxia Epidemica (aka, Vulgar Errours), Google Books Unto whom (what is deplorable in men and Christians) too many applying themselves, betwixt jest and earnest, betray the cause of truth, and insensibly make up the legionary body of error.

a member of the Legion of Christ, a Roman Catholic religious institute