This takes a turn for the worse when Casper whispers "Can I keep you" again.
Debatable, as the line itself doesn't have to have the creepiness that people tend to associate with it.
At the very least, it carries the knowledge that no, no he can't. He only gets enough time to dance to half of a song with her and to kiss her once before he fades away again.
He was also just saying it to tell her who he is. Mnemonic devices are a theme in the film (Casper's room, the pinky promise) so that was why that line was chosen.
At the end of the movie, The Ghostly Trio keep their promise to James and find Kat's mother, Amelia. Considering they've been nothing but jerks throughout the entire movie, they really Took a Level in Kindness in doing this.
There's also them scaring away Vic and Amber, who were about to crash Kat's party, although this was probably them getting a good scare out of it rather than wanting to help Kat.
But when you consider they didn't go and crash the party themselves...
In its own creepy way, the Ghostly Trio deciding they want to make Dr. Harvey into a ghost so he can hang out with them and be their friend forever... and then, moments later, deciding they like him too much to kill him themselves.