Sanae Kochiya is a human girl who left the outside world and moved to Gensokyo along with her entire shrine. While she is still a human, she is also considered to be almost the same as a god because of the faith people have in her.

When Sanae's ability to cause miracles is combined with Kanako's and Suwako's abilities, she gains the literal god-like power to freely manipulate the sky, the earth, and the sea. However, this power is difficult to use effectively as it only lets her cause purely coincidental phenomena to occur. The outcome could be good or bad. Happy or unhappy.

There is a simple reason why Sanae set out on her mission. She saw the warm smiles and words of gratitude directed toward Eirin and the others from all the patients they helped during the Gensokyo of the Dead event. This gratitude was similar to what the visitors to Moriya Shrine direct toward Sanae every day, but there was a fundamental difference that Sanae just couldn't quite identify. So Sanae decided to go and ask Kanako and Suwako for the answer.

Sanae wanted to save everyone who suffered from illness and make them happy by immediately curing any illness they may have with a single injection. That's why she decided to don a nurse uniform and fly around Gensokyo. This was only possible thanks to her ability―It was truly a miracle.

"I'll make you feel better right away..." Sanae was out on the engawa of Moriya Shrine on a clear, sunny day in the early afternoon. Kanako and Suwako reached out to wake up the sleeping Sanae, but they soon thought better of it. They couldn't bring themselves to disturb her happy sleeping face. It seems that the miracle that answered Sanae's question... came in the form of a very special dream.

Source: https://lostwordchronicle.com/lorepedia/story_cards/Mad_Sanae_Of_The_Dead