One could argue that the raincoat and the car were permanent secondary characters of sorts. Columbo is so attached to his raincoat that he can't do his job without it (remember in Now you see him he can't think without his partner the raincoat hehe)

It seems like Columbo preferred working alone. I remember a couple times when he found he would be working with a partner,it initially threw him off a bit until he got comfortable and tolerated it.

A cohort? ... nah - none were ever needed. At least, not my view.

That's why shows like Streets of San Francisco, Hunter, and Ironsides would never stand the test of time. In fact, I can't even sit through those shows today.

A couple served as intriguing adjuncts to Columbo (Kramer/Vernon). Truth is, I'd always thought of Dog as being enough of a character diversion, but often found him (and Dishy) far more irritating, than supportive.

I've also noted by suffering through a few eps that the pup never reappeared in the ABC installments. Hardly surprising to me though, because by that time, Falk was having enough trouble holding himself up!

I know Dog appears in at least one ABC episode. I don't remember which one, but it opens with Columbo at a basset hound gathering in a park. Dog wins his first, and only, ribbon and snaps at Columbo after he tells another gentleman that Dog is a gentle dog.

I know the ABC version very badly, but I'm sure I've seen that one. I'm pretty sure the killer is a painter, or at least it has to do with painting, so that should identify it.

Murder, A Self Portrait. Not one of my favorite episodes, but it does have Shera and Vitto in it and every time I watch it, I get an urge to go buy some loose fitting linen shirts.