In his first appearance in the episode “Cancer Man”, Ken mentions a number of what sound like firms, one of which he claims would “make (him) partner” just for walking in the door". Later, he suggests a bonus for himself which is “less than 10% of what I billed last quarter”. Sounds like a lawyer to me.
But next time we see him later in the episode, he talks about having a better attorney than someone he’s in a dispute with.
At first it sounded like he could be working for an investment or brokerage firm (which are big on bonuses), but they don’t really talk about “billing”.
He was a professional Bluetooth using, BMW driving asshole. So he’d be a lawyer or stockbroker, or any other professional whose car we’d like to see burn up.