Your subject looks translucent because they’re still being recorded during the long exposure after the flash fires. The flash freezes them for an instant, but any additional light hitting them afterward creates a ghosted overlay.

To keep the subject solid:

• Keep the subject as still as possible for the entire exposure.
• Make sure little or no light falls on the subject after the flash.
• Use a dark background and dark surroundings where the subject stands.
• Ideally, have someone else do the light painting while the subject stays in place.
• The light painter should move quickly and wear dark clothing to avoid being seen.

A stronger flash can help make the flashed portrait dominate the exposure, but it won’t fully fix the problem if the subject keeps moving in visible light.

If you can’t avoid movement, a practical alternative is to shoot two frames—one for the portrait and one for the light painting—and blend them later in post-processing.