In an interview with USGamer, Jerk Gustafsson of MachineGames discusses the challenge of creating a co-op computer companion that doesn't get in your way.

It's been really, really hard. But it's something that we... We knew it was gonna be tough, of course, but we decided to do it. We just had to do it, because we also want to tell this story. We want to have this. And that's part of what I said, we really need to rethink the approach to storytelling as well, with two players.

Since it's this relationship between the sisters throughout the story, talking to each other and the banter they have, and the communication they have, and that sisterly bond they build throughout the story is so important. So, obviously, we just had to have an AI companion if you play solo, because you need to have your sister there with you when you play. But the idea from the very beginning was to try to mirror the experience as good as possible between solo and co-op. So whatever you can do as a player, the AI companion can do the same thing. We do, of course, a lot of things behind this. How you switch weapons as an AI companion, that's controlled by certain requirements, depending on which enemy you face and what weapons they use on the player. But there are also some ways you can customize your AI companion and you can choose which pep signal you do.
Basically in the beginning, we just set out to do some emotes, like regular emotes. But I also wanted to add some g...