You can hire new Captains by collecting their fragments. You can obtain them either by researching Captains in the Academy or by using Summoning Scrolls in the Summoning Circle.
Once you have enough fragments to unlock a new Captain, follow the steps below to unlock and hire them.
  • Select the icon showing your Hero's avatar to open the Hero and Captain selection menu.

Select an empty Captain slot.

  • Once you've accumulated enough fragments to unlock a new Captain, that Captain will be marked with a red dot in the Captain selection menu, on the right-hand side.

  • Now click the Captain's info.

  • In the "Hire a Captain" section, click Summon to spend the fragments and hire the selected Captain.

  • The hired Captain will be automatically added to your selected free slot.

You can read more about the Summoning Circle and Captains' star levels.