The heart is the glue of the team. They usually serve as a driving force for the good of the team. They are the moral guide and make sure that the team is always moving in a positive direction. A perfect example of this is Ma-Ti from captain planet. With his powers, he knew where they were needed and will risk himself for what he feels is right.
While the heart can get over their head because they listen to their hearts without thought to the outcome of their actions.
Typically because of the nurturing nature and tendency of holding the group together the heart was a female and a love interest but as time goes on that is slowly starting to change.