Sir Markus
Keeper of the Watch
It's a semi-useful piece. When you select it at the beginning of the level, it brings up a menu of all the non-upgrade plants you have and you pick one. That allows you to have, say, 2 cherry bombs on a level so you don't have to wait as long for a recharge. The downside is that it has a delay on it--you drop it on the lawn as a potato and then it transforms into the plant you picked. So, where you can drop a cherry bomb into a pack of zombies and it will blow up before they eat it, with the imitator they will have enough time to eat it before it transforms and then blows up. I've probably used it most as a 2nd tall nut since the recharge on that is so long.
Alright, I get it. That could come in VERY handy with the Tallnuts, particularly on the rooftop levels, and on top of lily pads on levels with Snorkeling Zombies (they don't have to breathe, but they wear their snorkels because of peer pressure, but making them come out of the water makes them vulnerable to Cattails). Tallnuts and Pumpkins in combo are pretty cool and almost make the game too easy when you use them as a front line, but they do take a LONG time to re-generate, hmmm I'll try that Imitator; might be kinda fun.