Heidi is a kind and loyal hippo that has to go through a lot of tragedies. Throughout the first movie she has gone through being cheated on, depression and attempts at suicide due to the hate she gets. In the sequel she at first goes under a new name due to the rehab she went through but then after meeting Cesar, she went back to her old self due to a flier triggering old memories. 

Here is a reference for her voice to better understand her tones and such: https://youtu.be/4qF70aJYKdg

You must have discord as well as a decent microphone set up. As for other requirements being capable of singing with the voice and express emotion with the lines if need be. You do not need to be exact,  just recognizable to fit the character.