Eirin is known as the "Brain of the Moon" and is the head doctor of Eientei.  She's highly intelligent, but because of that she tends to act condescending and belittling to others who aren't up to her speed.  She's somewhat of a mentor to Reisen, similarly to Kasen with Reimu, however Eirin rarely shows anger and relies more on stern silence to stop an argument before it starts.  

Outside of her interactions with her apprentices, Eirin is rather jolly when it comes to those who are on her level, such as with her princess Kaguya or the upper circle of Gensokyo's big shots (The Yakumos, Remilia, etc.).