Reisen Udongein Inaba is Eirin's apprentice who lives in Eientei. There are many things about her that distinguish her from other beast youkai and youkai rabbits on Earth. She does not display any animalistic traits and is often seen genuinely interacting with and selling medicine to humans.

The insanity that Reisen can manipulate is related to the waves that are housed within something. If the waves are long, then that indicates leisureliness and a lack of motivation, leading to inaction. However, if the waves are short, then insanity ensues, emotional stability is lost, and the ability to hold a conversation with a person is lost.

The reason Reisen came to Earth was to flee from the war taking place on the moon. If Earthlings hadn't invaded the moon, Reisen might never have fled to Gensokyo, and she might never have met Eirin. She may have continued to help the Watatsuki sisters protect the Lunar Capital.

Eirin says that you have been a good girl this year and gives you the same clothes that appear in the story of the Gift-Giving Phantasm. They are incredibly warm and perfect for cold winters. They are so much warmer because they represent a level of appreciation that cannot be put into words.

Reisen and the others hear that Eirin has recklessly enacted a plan to give all of the good children of Gensokyo a gift. It is a chance to learn the importance of perseverance and setting high goals. In the end, everyone will be able to share a laugh over it... hopefully.