Firewall settings with "imagent" Among others that I recognize, I have the "imagent" been allowed to pass true my firewall (allow incoming connection). 

What/who is it, and do I must have it or can I just block it.

I do not remember allowing it in the first place anyway, but that could be due to my age :)
IMAgent is a process that listens for FaceTime invitations. It will be active even if FaceTime isn't running. You don't have to block it, but if you don't use FaceTime you can open the application, open preferences, and turn off FaceTime. That should quit the IMAgent process.