Dormmates are NPCs (Non Playable Characters) that you rent Rooms to in your Dorm Tower and increase your Strength and Intelligence stats.

To rent out a Room, go to your Dorm Tower. From there you'll see which Rooms are available for Rent.

Tap on any "For Rent" Room and you will see which Dormmates you can rent to. Each Dormmate has unique stats and looks.

Dormmates are separated into different Tiers. The higher the Tier the more powerful and expensive the Dormmate is. You can see  requirements for unlocking each Tier by switching to that Tier's tab.

Once you have your Dormmate you can upgrade them by spending Cash to increase their stats further. You can also Exchange them for other available Dormmates for a slightly reduced cost. To do that, just tap on your existing Dormmate and you will see the option to Exchange or Upgrade them.