what personality would the Knight of Heart have?
A Knight of Heart is one that fights for identity and self. The good thing is that we have three Knights and three Heart-bound, so we can use these guys to approximate something, although you must remember that YMMV on this, as Classpect does not equal personhood.
Looking at the Knights, we get Dave, Karkat, and Latula, and we can see right off the bat that they tend to have some self-confidence issues, and they often have issues with their own Aspect, like Latula having brain damage, and Karkat feeling mocked by his own blood. They also seem to cover up these issues with a can-do attitude.
Meanwhile, with Heart, we do admitedly have less basis on our claims, as Nepeta and Meulin aren't really all that important to the story, we can tell that Heart players often think a lot about relationships, and they often have trouble with their relationships, like Kurloz controlling Meulin, Nepeta having unrequited feelings, and all the bullshit DirkJake went through. They also tend to be very emotional.
So when we mash these things together, we get someone who feels inadequate about themself, and someone who also feels inferior about their relationships. They probably hide these issues to appease themselves, and they do a good job of it.