Well, when my character went up against a Haunter-morph, I made it so that it had a human body, but it was more like a balloon, as in, she cut it and the gas started seeping out. The extra claws appeared when it attacked and then faded away again afterwards. It eventually... deflated... and then exploded in one last attempt to kill her... *cough*
What I'm trying to say is that you have to keep the structure of the Morph in mind when you create the character, not that you have to make her a balloon... Just be careful not to make an uber-character, because they can always grow more powerful as we move along ;3