Batman: Arkham Knight unlocks most of the characters as you progress in game. But there might be a chance that you miss some hidden characters bio which you don't get in normal gameplay. So here is the list of all the characters including hidden, DLC's and their bio unlock locations in detail.
Hint:
Press Tab to enter the map screen and further press (-) to navigate to Character Bios screen. Here you can select and read them any time you want.
Characters available From the Start:
Characters available in Chapter 1: It’s Going To Be A Long Night
Characters available in Chapter 2: Patience Is A Virtue
Characters available in Chapter 3: It Is Dangerous Out There
Characters available in Chapter 4 & 5:
Characters available in Chapter 6: Some Kind of Bat Trick
Characters available in Final Chapter 9: The Die Is Cast
Characters available in Most Wanted Missions:
Characters available in DLC
Tip: Additional information is unlocked as Batman encounters important characters during his adventure in the main story and side missions.