That's a lot of reveals.
It was a big mistake to not kill the guy the moment he sensed his control slipped due to the host waking up because, as expected, that immediately lead to his opponents learning about him which means he's losing the advantage of surprise.
Then he just revealed himself to his host, which went well all things considered.
Then the stepbrother revealed himself as an enemy (though he's wearing a mask so maybe Stella won't recognise him, especially in the darkness, and Colon doesn't actually know who he is yet). Colon will be distracted keeping Stella alive so I guess there's a good chance the stepbrother isn't going to die immediately.
Finally, we see another parasite of, apparently, the same species though he's in a dog. So the stepbrother might very well be saved by them if he doesn't manage to escape. So Colon isn't unique.
I'm surprised the parasite is managing to talk with the dog; after all we learned that the parasite just manipulates the body, so what they can do with it is limited to its range of movements. I guess dogs do have vocal cords, but just lack the brain parts to use it like we do so if the parasite acts like the brain I guess that works.
I'm also wondering why, when the parasites can basically fully invade the nervous system, they don't just capture the signals directly from the body's own organs. So instead of making an eyeball pop out of one of the holes, they just see what the host can see by interfacing with their brains. I'm sure it's still faster to see with their own eye but it'd probably be a useful skill to be able to access what the host can sense. I guess that for us readers it's also important to show the parasite directly otherwise it's hard to tell one is there.