Ok, I have cleaned up your events and have it doing what (I think) you want it to do. I also duplicated your monster follow events to make some equivalent human follow events. This may not be what you want, it is just to show you the setup works both ways. You can find the capx attached.
A couple notes:
I noticed you had a couple every tick events as sub events. An every tick event which has any other conditions is redundant. Read this page to get more information:
I also noticed, your events to change which player is selected were set up in a way that they were firing every tick. With small project, you wouldn't notice, but as your project grew in complexity, it could really bog it down. I added an event at the start of layout to disable input on the monster and moved all of the events to switch which is controlled as sub-events to the space bar event. That ensures they only fire when you want to change users, not every tick.
I noticed a scroll to event at the end which, because of your camera seemed redundant. I am not sure why it was there. If there is a reason I don't know about, you will want to add it back in.
I gave the camera the ability to follow the monster when it is the selected character.
Finally, because you were using the lerp function to move the camera, I figured the easiest method to move the follower would be to use the same method. I figure if you know enough about it to use it, what I did should make sense to you.
As a bonus, I decided to help you cut down the number of events by 1 more. Where you set the selected value, you were adding 1 to the value. Then, in a separate event, you were checking to see if the value was above 2. If it was you were setting it to 1. This method works perfectly and is a good way to make this kind of change but eats up another event unnecessarily. So, I threw in a function you can use in your future projects. The method is "whichcharacter = 2 ? 1 : 2". This is a very simple if/then statement and can be used any time you have 2 possible results. This is what it means:
whichcharacter = 2 -> If whichcharacter is equal to 2
? 1 -> Then return 1
: 2 -> If not, return 2
Because I wrote the function in the action to set whichcharacter, the result is, whichcharacter is set to the return value. So, if whichcharacter is currently 2, whichcharacter is set to 1. If not, it is set to 2.
Well, that was a long one but I hope it helps. Good luck with your project.