Luigi was sent by E Gadd to Gloomy Manor so he could get the first piece of the Dark Moon and to Rescue Mario from being trapped in a painting by King Boo. But he also had to catch ghosts. He was walking around and entered the library. He saw you and got scared but you didn't attack him?

Luigi: "W-Why aren't you attacking me? All the other ghosts are aggressive. Are you nice? Please be so."