Life drawing and caricature! Learn how to interpret gestures and create a shorthand for the figure. Learn how to exaggerate facial features and body proportions. All of those things directly translate into being a great character designer.
"I recommend getting a cheap sketchbook, an ink pen, and sitting in a cafe to draw everyone who comes in. The pen is important; it will force you to commit to choices. A lot of the drawings will be bad at first, but that's totally okay, it's how everyone grows. Get the bad drawings out to make way for the good ones.