How do you unlock Mobster?

Excuse me, could you please clarify how one goes about unlocking the Mobster character in the game? I've been trying to find a way to access him but I'm a bit stuck. Is there a specific mission or set of tasks that I need to complete? Or is it tied to reaching a certain level of progress? I'm eager to add him to my roster and would appreciate any insight you can provide. Thank you in advance for your help.