A few years ago, I used to have a little monkey puppet in my act. I quit using him for awhile, but am now thinking about bringing him back for my brand-new show. Should I do this?

There's kind of insufficient information here. Why did you stop? Why would you bring him back? Why wouldn't you bring him back? What purpose did he serve before? Do you still need that purpose served? Does he actually add to the show, or would he be just one more thing to do? What's the show about, and how would the monkey fit it? Can I think of any more questions?

Anyway, you get the idea. No character should be in a show, just to be there (imho). If the character has real purpose, then by all means, bring him in. The purpose might be something as simple as "I really like this puppet character, and interact well with him, and he gets my performance juices flowing." Even so, the character should not distract from the rest of the show.