Your 25 liter(more than 6 gallons) is perfect for a single Betta.
Get a short fins and tail Betta.
Avoid big/long fins and tail Betta that often have fins rot.
You will need a heater and a small filter such as sponge filter or hang on filter.
If you use a hang on filter, you need to cover the intake with a strainer sponge to prevent it from sucking in the Betta fins/tail.
If not, you can get a group of Cherry shrimps, probably 10-20 of them.
But make sure that your tank is fully cycled and have some algae.
When I was preparing my tank for Sulawesi shrimps, I prepared my tank for at least 3-4 weeks for the algae to grow.
During that period, I turned on the aquarium light for at least 10 hours per day.
But for Cherry shrimps, they don't need so much algae as they will eat shrimps pellets.
Here are some videos of short fins and tail Bettas.