I found Ed from Cowboy Bebop on my first playthrough, but I don't know what actions triggered the event. How can I get the rest of easter egg characters? And also do I have to find them in the same playthrough to get the Steam achievement?
I found them all, they are the main characters from Cowboy Bebop:
Spike: When you get into Matthius apartment, after the Boryokudan members leave and before checking the terminal return to the first screen.
Jet: When you can change between characters, go to Hibiscus Highrise and he is in the second to last level of stairs.
Faye: After curing Paul Erickson, return to the burned down hideout where you obtained the Carbon Ray Stabilizer.
Ed/Ein: As soon as you can get in the appartment building where Paul Erickson lives, go to the top floor and you will find Ed next to the window.
And you need to find them in the same playthrough to get the achievement.