The good ol’ surprise threesome. We need a full list. I apologize if I don’t have names for some of these characters, I am too lazy to go back and look.
Zombie exodus safe haven: If you are romancing Jillian, when you reach part 2, and you are in your tent with her, about to get things started, Gina will join you.
Choice of robots: AS mentioned, if you have a wife, and a robot who I guess is human like, you can play together.
The last Wizard: I don’t remember the character name, but the hot blonde guard who saves you when you first arrive. If you end up with her, she will eventually bring a friend along to join you.
The Eagle’s heir: Didn’t achieve it, but there is one in there if you check the achievements.
If anyone has others, with as much specifics as possible, copy the above and then add to the list. it seems the event only occurs with a specific character, in specific circumstances, so simply mentioning the title may not get you far.