Dan said adding pets would be tricky. He had a few strategies, like putting pets beside, on, or away from their owner. Another one was having pets only appear when they attack, though he said that wouldn't be very popular.
First, let me clarify that I'm referring to active pets. Ones that attack, level up, have skills, etc. In other words, not the ones that just follow around behind you. We'll have those too most likely but they are much easier to implement for obvious reasons. Another thing is every character can essentially have a pet because it'll be a class skill and as I mentioned in another post your class is dynamic based on the skills you choose. So you could basically put one point into the pet skill and now have the ability to fight with a pet. Granted you'll need to invest a lot of points in the pet skill and probably other pet related skills to make your pet feasible as you level up. In other words, you can easily have a pet but if you don't invest many points in your pet skills it'll be rather lackluster.
And finally the last idea was to have the pets literally be characters. The first game that comes to mind where they had this is Dragon Age. Basically, the pet would be one of your 5 active characters that you have out on the field. This is a slightly different route than the other solutions. I wouldn't expect them to have armor and weapons really or maybe they'd just have equipment that doesn't display (we don't want to drive Kevin insane). Also they wouldn't be nearly as customizable as the other characters but they would have skills and such, maybe some skill points that can be assigned as they level up, etc. So think of them as just being more specific and less customizable characters. This might sound like they'd be lesser characters overall but I'd expect them to be quite strong, just more limited. You could maybe even have special combo moves that can only occur when you have certain pets together. For example, maybe you have one character and 4 wolf pets. Maybe you'd get a special passive skill called Pack Leader that gives all your units a nice buff. Just something like that perhaps.