There are three other secret characters to unlock. Here are their names and how to get their addresses.

Moonstone: You will see him in some of the Lambda server areas. Later, you will get an e-mail from him. You will go to Lambda: Bottomless, Guwaffing, Raw Ore with Moonstone (auto) and someone else. It would be prefered to take a Wavemaster along.

Nuke Usagimaru: Read the board after completing Delta: Soaring, Sky Bounded, Abyss. You must go to Lambda: Lightless, Distant, Dead Lands alone. I suggest training before you go.

Rachel: Read the board after completing Delta: Soaring, Sky Bounded, Abyss. You must go to Lambda: Collapsed, Pagen, Remains with a random party of three.