User becomes stronger, faster, more durable, etc. when they come into contact with sunlight, possibly unlocking abilities related to the affinity and enhancing the existing powers. Some users may be able to draw sustenance from the sun or even slow or stop aging.
Kayko Yako (Ninjutsu series); via Burst of Phoenix technique
Solaire of Astora (Dark Souls)
Leona (League of Legends)
Castform (Pokémon); via Forecast ability
Pokémon using "Solar Beam" or "Solar Blade" (Pokémon)
Pokémon using Fire-type moves (Pokémon)
Pokémon wih the "Solar Power", "Flower Gift" and "Chlorophyll" abilities (Pokémon)
Kotal Kahn (Mortal Kombat X)
Gawain (TYPE-MOON); via Numeral of the Saint
Protagonists (Fallout series); via Solar Powered perk
Crenshinibon (Forgotten Realms)
Firebenders (Avatar: The Last Airbender) utilize the sun to fuel their bending.
Superman (DC Comics) gains his powers by being in the presence of a yellow or blue sun.
Supergirl (DC Comics)
Like his cousins, Lar Gand/Mon-El (DC Comics) gains his powers by being in the presence of a yellow sun.
Starfire (DC Comics)
Due to having her DNA mutated into a plant-like capabilities, Pamela Isley/Poison Ivy's (DC Comics) will live longer and aged slower than regular humans, as long as she maintain absorbing enough solar energy.
Virgil Hawkins/Static Shock (DCAU) has this ability via sunspots.
Apollo (Wildstorm/DC Comics) can only retain his powers and health from exposure to solar energy.
Nuova Shenron (Dragon Ball GT) has an affinity for the sun.
Ultraman (Ultraman) has an affinity for the sun.
Sunspot (Marvel Comics) gains his solar based powers from the sun.
Diwani Alvera (Enchanted Garden)
At nighttime, Escanor (Seven Deadly Sins) is a scrawny, weak little man...
...but once the sun appears, his Sunshine powers activate and make him a godly powerful warrior...
...and at high noon, for one minute he becomes the invincible The One.
Stella (Winx Club) Guardian Fairy of the Shining Sun, Moon, and Stars.
Gigalith (Pokémon) can absorb solar energy into its energy core and release the energy as a powerful ball of light.