One character that means both 'sky' and Buddhist 'emptiness'. Exceptional choice.

What a Japanese person actually thinks
Genuinely admired. Japanese people find this character beautiful precisely because of the double reading — openness and nothingness being the same thing. A foreigner choosing it reads as someone who understood something.

Meaning

Literally: empty; sky
Actually: Read *sora* it's the sky. Read *kū* it's śūnyatā — the Buddhist emptiness at the heart of the Heart Sutra. Same character, and the doubleness is the point.

Scores

Naturalness●●●●●

Extremely common word with real philosophical depth behind it.

Coolness●●●●●

Simple enough to age well on skin, deep enough to be worth explaining. That combination is rare.

Watch out

lowAlso means 'empty' in the mundane sense — 空車 (vacant taxi), 空席 (free seat). Nobody will read a tattoo that way, but it's the word's day job.

On skin

空

Orientation: either

Mincho or clean brush. Eight strokes with a lot of white space — it will still be readable in forty years.