Witch: Someone who manipulates, changes, and breaks the rules of their aspect. They also seem to have a tendency to work with or for powerful forces that have a great deal of power over their aspect.

Heart: Basically soul, the essence of someone's being, and someone's emotions. It's speculated that it's also related to intuition. Heavily associated and likely directly tied to romance and relationships.

Well, alright, I don't think there's many assumptions you can make on someones personality through this class. Not anything extremely specific, anyway. But the big thing about a Witch of Heart is that they will have a huge impact on the emotions of the people around them in general. As an extension of this, they'll also likely directly interfere with the relationships of the people around them.

They'd absolutely share the powers that Dirk has, but won't be as confined in the way he is, and obviously wouldn't destroy their aspect within themselves. Destruction is a form of manipulation, after all. But they'd be able to do a lot more than that, and change feelings within people as well.

Their actual powers are pretty simple. They fuck with peoples emotions. They'd start out doing this naturally, but as they grow stronger, they could change peoples feelings about things and even warp their intuition without a word. Basically like a less overt form of mind control. They don't manipulate thoughts, just emotions. An extremely powerful Witch of Heart would be able to actually change someone's heart, which is just another way of saying that they can manipulate someone's soul.

Forcing someone evil to have a change of heart and become a better person would be them breaking their aspect if there's no logical reason for that person to have actually changed. Of course, if the witch isn't exactly a good person, they could also do the opposite. Their powers would depend entirely on how suggestible their target is, though. Someone with a strong enough mind and enough conviction in their beliefs may not be able to be swayed very easily.

As for the tendency for a witch to get involved with a powerful force of some kind, I'm not sure. A Muse or Lord of Heart from another universe is always a valid option. But this isn't something a witch necessarily has to do, it's just something consistent with the witches in homestuck (Jade is involved with Alt Callie, Feferi worked with the Horrorterrors, and Damara worked on behalf of LE).