Reads like a costume. The single character 忍 is much better.

What a Japanese person actually thinks
Cartoonish. To a Japanese reader this lands roughly where 'PIRATE' would land in English: a real historical thing that has been thoroughly absorbed into children's entertainment.

Meaning

Literally: endure/conceal + person
Actually: Ninja. The historical operative, and — far more commonly — the pop-culture character.

Scores

Naturalness●●●●●

Normal word.

Coolness●●●●●

The 者 ('person who') suffix is what does the damage — it turns a quality into a job title, and the job is a theme-park one.

Watch out

Nothing embarrassing lurking in here.

Get this instead

忍 nin / shinobu
endurance; forbearance; to bear in silence

Drop the 者 and the whole thing changes. 忍 alone is a serious virtue — enduring hardship without complaint — and it's a genuinely respected character. Same visual, completely different register.

On skin

忍者

Orientation: either

Brush, especially if you take the single-character option.