Moero Onii-chan

For the first 100 years I was as gentle as possible ... However, humans spent these years fighting each other.
For the next 200 years I was rough. Humans hated me and tried to distance themselves.
Next 300 years I spent in silence like a stone. Some humans praised me as God.
Over the next 500 years I listened to the people and occasionally fulfilled their wishes, but hardly received any gratitude.
So I quit everything. Counting time ... thinking...
I forgot about birth, purpose of life and even name.
However, humans call me "demon" ...

Main character is a very strong demon who lives in a solitude in the mountains. However, recently a group of lively girls has arrived. They bring a lot of fuss and rely on demon onii-chan help.