The Tyrant or Mr X as he is called sometimes is a haunting character who will follow you relentlessly throughout RPD trying to kill you and if you aren’t careful enough he will find you and attack you without notice. So it is wise to avoid him as much as you can. There are ways in which you can halt him though.
Can the Tyrant (Mr X) be killed?
As much as you try to avoid him and get rid of him there will be a moment when you start thinking about killing the Tyrant because doing so will afford you to play the game at your own pace (you will miss out on the S+ Rank though).
But as relenting and unforgiving he can be, no matter what you do he cannot be killed, but fret not as he is really stiff so if you manage to sprint away from him, you can gain a lot of ground ahead of him.
How to avoid the Tyrant
The Tyrant seems to have a sharp sense of hearing, you might want to kill off zombies that come your way as silently as possible, with that in mind equip your guns with a silencer or use knives as much as you can. He will find you if you wait at a particular area for too long and you will have to outrun him again so it is best if you keep moving and be on the lookout.
How to stop the Tyrant
Although he is immune to physical damage, you can easily halt him from coming towards you by shooting at his face and he will stop in his tracks giving you enough time to run through multiple doors and get rid of him for time being. Another thing that you can do to is, keep continuing on the mission at hand as that is the only way you can stop him.