I just entered Ding Dong Well and got Thumbelemur. At first I was certain that he needed to level up and he would be great, but it seems in every battle if I use him instead of Mitey he takes ~15 damage and his attacks don't seem to stop any specials the monsters use.
So.. Does he get better? Both Mitey and Thumbelemur are level 8, and Mitey is 10 times better in battle.
In the early game the lemur is a change of pace attacker you use to interrupt slow attackers with slow developing attacks. By third transformation, the lemur gains useful healing spells and completely changes roles.
The lemur of doom is pretty useful. Psyche up is decent for interupting long casting attacks ((The boss of the sewers never got anything off thanks to it)) and he attacks really fast overall. Mite WILL take a good deal less damage though. Overall I am more fond of Mite, but both will do the job.
I preferred the lemur right when I got it since it was so fast, but Mitey overtook it in damage pretty quickly. I got rid of Mitey fairly early on (traded him in for a Purrloiner), and have since leveled the lemur to its final evolution. Now it's a lackluster attacker with good evasion and decent accuracy and I'm looking for something to replace it. I kind of wish I had stuck with Mitey since I get all sorts of armor and shields later in the game and I have no one to put them on.