Sylph: Sylph’s are similar to witches but they manipulate their aspect in a more abstract way, with a focus on healing. They can manipulate their aspect to heal it, making it thrive in their session. A common way to do this is to manifest their aspect into others, or healing it within them. Their aspect is something that’s fragile or flawed in their session, and they always take it as their duty to both nurture it and fix it personally.

Heart: The soul itself, the essence of someone's being, and positive emotions. Directly tied to relationships and romantic feelings. It's speculated that it's also related to intuition and making decisions based on gut feelings (opposed to mind, which deals with decisions made based on logical thinking).

So for this one I'll start out with what I think a Sylph of Hearts literal powers are, as I feel they'd be pretty straightforward. Sylph's manifest or heal their aspect within others. A Sylph of Heart's most central and prominent ability would be manipulating peoples romantic feelings towards others. They could do this in a lot of ways though, sure they could roleplay as cupid, but in an instance where a relationship between two other players is facing difficulties and they're growing distant, the Sylph would be able to close that distance, and heal their lost passion for one another. Or if one party in a relationship is taking the other for granted or mistreating them, the Sylph would be able to fix that and make them change.

If someone's soul itself was hurt or broken, a more powerful Sylph of Heart would be able to heal it, bringing them back to reality and helping them feel like a person again. You use this idea in different ways, as the concept of soul itself is always left mostly vague in the comic. However it's important to note that Dirk's mental and literal splintering of himself was him destroying heart within himself. This leads me to think that the most basic way someones soul/heart could be broken is anything that causes a disconnect from reality or their own person-hood, dissociation or the general feeling that they aren't a real person, hence the first sentence in this paragraph.

As I've said at least once before, I think physically touching someone would be required for a Sylph to use their powers. If I'm misremembering that, let me know, the only examples we have are Aranea healing Jake and Terezi to my knowledge.

Since a Sylph's aspect is always fragile or flawed, their session would probably be rife with awkward or troubled romantic relationships and yearning, and possibly even a lot of upsetting emotions in general (though that's a constant with most sburb sessions I'd imagine). They might see themselves as wanting to be a relationship counselor, a matchmaker, or they could just be someone who wants to make sure everyone is happy, which would lead to them making sure they're okay in their relationships. You could absolutely get creative with why and how they go about taking it as their responsibility to heal and nurture heart.