Yasashii Akuma (The Kind Devil)
An Amazon Prime Japan Video exclusive.
Foo, a kind boy, lives together with his grandmother. However, Foo has no friends. One day, as Foo is off picking herbs on a mountain to treat his grandmother's illness, he finds himself lost. But then he is saved by a boy demon with bright-red skin named Smoochy. The two kind yet friendless boys meet and become each other's first friend. The two begin to spend their days playing together, until one day Foo suddenly stops coming to the mountain.
[Source: Official YouTube channel]