Pack breathable layers and a rain layer for spring/summer, add warmth for winter, and always include shoes easy to remove, cash, and transit tools for smooth Japan travel.
Clothing
Lightweight tops (breathable layers for humid summers)
Long-sleeve shirt or cardigan (for sun, air conditioning, and modest temple visits)
T-shirts or base layers (easy to mix and rewear)
Pants or jeans (comfortable for cities and train travel)
Shorts or skirts (best for summer; modest lengths for some temple areas)
Mid-layer fleece or sweater (spring/fall and cool evenings)