There are some easy ways to make a character cold and calculating, as you want.
Example: "The odds of your surviving this are twelve percent." *stabs*
But that's a bit too easy for my taste. So, let's see what we can do about that. Instead of just stabbing the guy he (or she) is after, he can stab through his ally and into the intended victim. But that doesn't really maintain the calculating part of the character.
Guns. That's it. Make him a crack-shot with a rifle, and have him shoot through some civilians to lodge bullets into the good guys.
Excitable and psychotic seems easy enough, but why bother talking about that when you could simply read some Black Lagoon? Pay particular attention to Revy if you do: she is essentially the character you want.
Quiet and follower. Well, you could go the easy route and just have the character be completely mute, making them incapable of leading, but that's just a bit too easy.
What if you try getting someone who has (or can make) a somewhat wispy voice, breathy and barely audible? The few times that character talks, you can tell that they prefer to stay silent. Make them passive as hell, flinching whenever someone shouts or screams, for example.
For the bold leader with emotional problems, I've got nothing. Sorry about that one. Maybe make him/her a heavy drinker who only reveals his/her problems while drunk? I really have next to nothing on this.
Most of these ideas are subtle, but usable for a vlog. They'd probably work best in a narrative, but they can work in a vlog. A couple of the ideas could easily be swapped out for something better, but most of them are at least decent.
Good luck with your project!