Wow this was quite scary, especially when I triggered the follower for the first time. The music and the sound effects really contribute well to the horror. Definitely left me with questions about what the follower is and who we are.
Mechanics wise I think calling the follower was a pretty good mechanic. It seemed like you only ever needed to call the follower once in a given level but I think you could design levels that required calling the follower multiple times.
I think it would have been cool if there were some levels where the follower patrolled on a fixed path so you had to do time when to sneak past. I'd also love for there to be some unsettling messages -- perhaps documents dropped by former scientists at the facility or something.