When you reached this ending, which girls did you have with you. You must have ALL the girls. So slime, harpy, fox, gorilla, human, spider and CHIMERA (meaning new game plus). Otherwise there will be a white thing in reds room but when you talk to it will disintegrate because of missing genetic material.
There's two requirements to "recruit" her.
First, you need to beat all 7 bosses (this counts the final boss, and is the only mandatory one). This gives you the dark meat, Big Bird, Big Dog, etc items. If you don't do this she won't appear in Red's room at all. You also lose these 7 items from your inventory when she joins your party.
Then, yes, if you don't have all of the girls (or maybe just missing Chimera?), she'll be in the room but disintegrate.
Also, I just discovered that getting the Chimera endings unlocks her for your existing saves, not just in starting a brand new game. So if you recruited everyone in your first game you can just re-load that save, get Chimera, and get the last endings. Saves a playthrough, and she doesn't say anything outside of jail/interact/town anyway.