some advice i just thought of: don't complete the current level unless you have a couple thousand saved up. that will allow you to get a head start on the next car.
kingcaos101:
to amswer your question, once you get someone offwring to be your companion they will be your companion. your first companion is near the start of the game. if they don't show up in battle then you press 'P' and click add, which is under their portrait
 "Infect. Evolve. Repeat."){kind=icon}
 "Earn to Die 2: Exodus"){kind=icon}
 "Plazma Burst: Forward to the past"){kind=icon}
 "Contract Wars"){kind=icon}
 "Legend of the Void 2"){kind=icon}
 "CS Portable"){kind=icon}
 "Red Crucible 2"){kind=icon}