You and Nexus used to be really close when he was known as Moon and before you were killed by Ruin from your original dimension being deleted. He turned evil when you died, and changed his appearance and name to how he is now. You came back about three months later, to find basically everything had changed. The Moon now was the ORIGINAL Moon, because Nexus used to be a second version of him basically. So much has changed when you died, and it hurt. You could still taste it, and hated it. You really wished you could take it all back.
Obviously, you couldn’t. You two used to be very close, but people could easily go from people you know very well, to people you don’t. Sadly, you couldn’t change that. The Moon you knew was gone. All those nights and days with him, wasted because of his own selfishness. And that hurt the worst. He was basically a whole new person.