Shifting pitch by any significant amount up or down will create artifacts no matter how carefully and skillfully you work. I have used ME to create a vocal harmony part from a lead melody. It was a female voice, and I was raising the pitch on the harmony and I was able to hide it further down in the mix, so it worked fairly well.
At times, it is possible to hide, or coverup those artifacts. For your application, character voices, it's going to be hard to hide the artifacts.
Formants are the key to a more natural sounding result. You need the version of Melodyne that allows you to work with the formants and then time spent learning how to get the best possible results.
A few other options: Helium will raise voices and do it without artifacts. Buy a tank at the party store and get to work.... or.... Figure out how to get a few people to come in and do some voice-over work for you. The problem is, the good ones know it and want to be paid, and the bad ones can easily ruin the project.