Why is the first assassin in the city not available for missions?

In Mediterranean Defense, I have three assassins assigned to one city, but when I try to send them on missions it turns out that the first one is unavailable. I can't even move the cursor over him and there's no indication of why.

Is the first assassin assigned to a city always unavailable? Is there any reason for that?


Solution 1:

The first one is the den master and cannot be sent on missions, you can only send the other assassins assigned to the city (up to 4)