There is no such thing.
A "PURE" heart I would think would be one that is untainted by anything. Since the definition of pure us to be unmixed. So a pure heart would be one that never sinned or has ever let darkness in. To find a pure heart you need to find someone who knows nothing of the world and thinks only of rainbows and things.
now if we want to be more kind and go off what most stories will use its simple some one who hasnt let evil in and although this may appear to be the same as the above its not. In stories the person who is pure of heart often struggles because it is hard to stay that way while living in this world or the world of most stories.
Children are often seen as the pure ones but the reality is even after a certain age not all kids are pure and this pure(ness) is destroyed but those whose hearts are already corrupted.