There are four different playable characters in Gotham Knights, Batgirl, Nightwing, Robin, and Red Hood. Who you wish to play is entirely up to you, but you can change it if you want some variety. Here's everything you need to know about how to switch characters in Gotham Knights.
How to change characters in Gotham Knights
To switch characters in Gotham Knights, you must return to the Belfry and interact with one of the suits displayed in their cases to the left of the batcomputer. Just walk up to whatever character you want to switch to, hold the interact button, and you will change into that hero.
There is currently no other way to switch heroes in Gotham Knights. You cannot do it during your nightly patrol, you always have to return to the Belfry and end your night patrol. If you want to flair up the Belfry with some aesthetic style, the suits displayed will always change to the suit you wore the last time you played as that character. It's a small but cool touch.
For more Gotham Knights guides on Pro Game Guides, check out How to unlock the fourth ability tree in Gotham Knights and How to interrogate enemies in Gotham Knights.