(No idea if I spelled that right)
When you go to the MTT Resort, the "chicks in the alleyway" or that alligator and cat have a key to someone's house...
Get that key and go to the house beside Nabstablook's.
Read all the dairies inside it.
Then, once you beat the game, go back to Nabstablook's house and you will see Mettaton EX.
He will say that" Bl- Nabstablook is joining me. He will be the DJ." (Too lazy to find the lines)

Theory:
The diaries state that Nabstablook's sister left him to follow her dreams. She kept calling Nabstablook "Blooky" in her entries. Later on, Mettaton EX says Bl- the first two letters of Blooky.
Is Mettaton actually Nabstablook's sister?

Who else noticed this?