Why is it that of all people it's Perkins, a FBI agent who is sent to talk and make a deal with Markus?
I'm not even asking this time why can't we ever talk to a single journalist, but just...is it something FBI would be dealing with? What kind of rank does Perkins even need to have to be this high up?
You mean outside of the fact that Cage just really likes the FBI and really, really doesn't understand how cops work?
In all honesty, I do think that the main reason why an FBI agent would be sent to deal with Markus in any capacity is because Cage wanted to do some Norman Jayden-esque stuff with the plotline. We know that Connor is based off of him and that three of the original four characters had direct one-to-one ties with Heavy Rain's various protagonists, so it's not surprising that he'd want to add them in. And while Perkins' personality is more Carter Blake than anything else, I do see a lot of Norman in him for me to think that perhaps he and Connor shared an inspiration character at one point in their respective development processes.
As for the question of if the FBI would actually get involved in these sorts of cases... I mean, maybe? The FBI is an investigative agency that deals with, amongst other things, cyber crime - which Jericho's attack upon the Stratford Tower might fall under. There's also the issue of it potentially being a white-collar or financial crime if CyberLife was involved in any way, and the FBI does cover that too. But primarily, the FBI deals with violent crime and criminal activities that either cross state lines (to make it a federal investigation versus the state's) or if the local police department called them in for an assist.
So long as Jericho's actions don't reach an international level, then they'd probably just be dealing with the FBI - hence why they deal primarily with them throughout the game. From my very brief Google search, I think that 'transnational' threats would be deal with by Homeland Security, so if Jericho ever went global (or was a global organization to begin with) then that's who would be in charge of the investigation. So technically, it is accurate to send the FBI as they do in game.
The only issue that I have is that the FBI primarily doesn't work with the military to the degree that we see Perkins do in Crossroads and the Demonstration. It might be one thing if Perkins was working in an advisory or consultant capacity, but he's actively shown giving orders at the end of Crossroads...
Soldier: They detonated an explosive in the hold. The ship is sinking, sir. Perkins: Order the men to evacuate. Soldier: Calling all units! Abandon ship and evacuate immediately! - Soldier & Richard Perkins, in Crossroads, if the explosives were activated
Soldier: We found several hundred androids in the hull, sir. Perkins: Take 'em away. Soldier: Yes, sir. Perkins: Then clear the ship and order the men to withdraw. - Soldier & Richard Perkins, in Crossroads, if the explosives weren't activated
...so he's clearly working with them as a commander of sorts. Which, again, doesn't makes sense but can be chalked up to Cage not knowing how the FBI works and/or Rule of Cool.
As for why Perkins was chosen, we actually get an explanation for that in his Gallery Bio:
Basically, Perkins is one of the top agents in the Bureau and is highly respected by his superiors, and has demonstrated his reliability in the field before. He also is willing to work the cases that no one else wants because of the sensitivity of their nature, which means that Perkins may have been the only one willing to take the Stratford case on.
He does seem to be more of a Supervisory Special Agent than just a run-of-the-mill FBI guy like Jayden was. Because of Perkins is the one running point on the FBI's investigation as his station amongst the military supposes he might be, then he's probably the one in charge of the much larger team of FBI investigators that we can see wandering around the broadcast room in Public Enemy.
So at the very least, Supervisory Special Agent is probably his ranking, and he has a team of agents that operate beneath him as well as his own FBI android that we never really get to see in game.
