'Quiet warrior' translated adjective-first. It reads like a fantasy character class.

What a Japanese person actually thinks
Reads like a class name from a role-playing game character sheet. Not embarrassing, just entirely fictional in flavour.

Meaning

Literally: quiet + warrior
Actually: Grammatical, and instantly recognisable as an English phrase converted word by word. 戦士 also skews toward RPG 'warrior' rather than anything historical.

Scores

Naturalness●●●●●

Correct grammar, no idiomatic basis. The な makes it visibly a translated adjective phrase.

Coolness●●●●●

Five characters with hiragana in the middle breaking the block.

Watch out

Nothing embarrassing lurking in here.

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沈着 chinchaku
composed; unruffled under pressure

The quality you meant by 'quiet', as a real word.
静中動 seichū dō
movement within stillness

A martial-arts concept: complete calm holding complete readiness. Three characters, and it's exactly the paradox 'quiet warrior' is reaching for.

On skin

静かな戦士

Orientation: vertical

N/A — change the words.