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I don't have any experience with it, but this Android app has a real-time voice changing function (as near as I can tell, most apps like this will record your voice and then play it back, rather than changing your voice in real time). Looks like you'll need speakers or a headset or something—this is probably intended to limit feedback.
Here are some other options from other developers:
RoboVox Voice Changer on Google Play (android) is another. It has a real-time mode as well.
On iOS, you can try VoiceMod - it has a few voices that can be used for characters. By the way audio latency is way better on iOS (but Android is improving).
There are some real-time voice changers from other developers as well.