There's a fanfiction called 'Pikachu's Promise to Buneary'. It starts with Pikachu making a promise to protect Buneary during a thunderstorm. This simple promise sets off a chain of events where they go on little adventures around their Pokemon world. The author does a great job of describing their surroundings and how Pikachu and Buneary communicate without really using words.
I really like the fanfic 'Pikachu and Buneary's Bond'. In this story, Pikachu first meets Buneary when she is injured. He takes it upon himself to help her get better. As they spend time together, they form a deep bond that goes beyond just friendship. It's a heartwarming story that shows how Pokemon from different species can connect on an emotional level.