The first question is not whether Japan is safe
JNTO’s solo travel guide already makes the main point: Japan is highly suitable for solo travelers because of its public transport, signage, day-to-day infrastructure and social acceptance of doing things alone. The better question is what kind of solo trip you want.
Choose cities by the kind of solitude you want
Tokyo is best if you want freedom without isolation. Kyoto is best if you want reflection and cultural depth. Osaka is best if you want easier evening energy and a more casual city feel.
Keep the route simple enough to feel free
The best first solo Japan trips usually have few hotel changes, clear city bases, a mix of booked structure and open time, and evenings that do not require maximum energy every day.
When to get personal help
Personal advice is worth it if this is your first solo Japan trip and you are choosing between Tokyo, Kyoto and Osaka, or if you want accommodation that feels safe and convenient without becoming dull or overly isolated.