Why is Avalokiteśvara the "main character" of the Heart Sutra?

I'm a Zen practicant and, as you may know, we recite the Heart Sutra of the Perfect Wisdom a lot. I really like this sutra, it is very meaningful. However something I never understood is: why is the Boddhisattva Avalokiteśvara present in the sutra? The sutra teaches about vacuity and originated dependence etc. And, as far as I am concerned, Avalokiteśvara is usually associated more with compassion. So why is *he* specifically in this sutra?