For me, I started off using Johnny Gat and basically beat the game and most activities using him, which most people I assume would do aswell. If you wanna know how much time you used this person, you can go into you Quest Log and look under "Miscellaneous". There is a challenge that says how much time you played as that character. If you don't have the challenge, then you have completed it.
To switch characters, go to the Ultor Tower HQ where you will find either Johnny or Kinzie in the lobby. (Depending on who you are playing as)