A Bard of Heart would start off acting like a Mind player. The opposite aspect of Heart is Mind, which represents logic, false faces/outer self, planning, memories, and judgement. So this Bard would be a very logic-oriented person, never allowing their emotions to get the best of them; this would be clear from their acute decision-making process. According to the Hiveswap sign test definition, Mind players also have a fluid personality capable of changing to fit situations. It's likely that this Bard would employ this kind of adaptable personality to the fullest, varying their behavior and interests from person to person so they will be liked or at least accepted. They wouldn't be very open about themself or their true desires mostly because they aren't even sure what those are. It's likely that if they weren't in a situation where their personality could be conformed to suit a certain person, they would be apathetic and deadpan.
They might think that their own opinions, if they have any, are unimportant or stupid. Maybe they feel as if being emotional is dangerous and unpredictable, unlike the cold logic of Mind, so they bottle it all up and stow it away in some dark corner. They let their feelings essentially get stomped on because they don't feel as if they should stand up for a certain opinion. They would also be avoidant of discussions about feelings or just act like they don't care at all. Mind also represents blending in with the crowd, so this Bard might just absorb the mannerisms of other people in order to look like the others, without really having a passion for anything on the inside. Deep down, the Bard doesn't really know themself.
Bards do start out acting like players of the opposite aspect, but once they have a sort of collapse in their life, they actively pursue their aspect, trying to take in as much as possible. The Bard of Heart might suddenly be overcome with passionate feelings about something or someone. Maybe it's a cause near and dear to their heart. Maybe they fall in love for the first time. Alternatively, this could involve the part of Heart that involves the self. Perhaps they were noticed for something about themself that they didn't want anyone else to know. Or maybe they were revealed to be a supposed 'fake' due to their fluid personality.
The Bard of Heart will begin to question everything about themself. Are they really invested in this thing all their friends like, or are they just deluding themself? I could see the Bard having one big identity crisis, asking themself variations of the same question, over and over: "Who am I?" They don't know. And suddenly, it becomes an all-consuming quest for them to find out. They would find themself becoming passionate and biased, no longer afraid of standing out. They'd wouldn't let their feelings get stomped on anymore, either.
The Bard eventually creates a sort of balance by destroying their aspect, intuitively learning "how much is too much?" They'd be able to allow destruction through Heart. This could involve them being a manipulator of others' feelings, only to break their hearts, leaving a mass amount of distraught people in their wake. The use of emotions for destructive purposes could also involve gradually stirring up a crowd with their abilities until everyone is full of furious passion and a fight breaks out, turning the enemies against each other. If the Bard were more evil, they could use this against their own team as a distraction. Another way this Bard could destroy through their aspect is through themself, or their soul. Imagine you're in the dream bubbles and this Bard summons up every spirit version of themself to attack their enemy. Wild, right?
Bards also allow the destruction of their aspect, so the Bard of Heart might literally be able to destroy the soul. Not in a flashy way like Dirk (Prince of Heart) does it, though. It would be more subdued, perhaps a gradual decay. Maybe that would be more terrifying. Imagine: Soul rot, in which someone's passions, emotions, and sense of self are gradually withered away until all that is left is a cold, unfeeling shell. Terrifying! Besides that, the Bard of Heart could destroy emotions, which could work for better or worse depending on their nature. Prince are active and are therefore less capable of pinpointed destruction, whereas a Bard's destruction is more like a gradual enervation. So they'd be better than a Prince of Heart at obliterating specific emotions.
As for a land for the Bard of Heart, I suggest the Land of Tea and Gondolas. LOTAG literally runs on logic; the consorts ship tea around in cute little gondolas that go on the land's rivers. The tea strengthens their minds and makes them excellent decision-makers. However, this is starting to change; a new type of tea has come out that incites passion and emotion…at the cost of the consorts going on rampages! The Bard of Heart must cause the gradual decay of this item's popularity using the influence of emotions, as well as the destruction of some of the gondolas, but they must be careful not to destroy too much, as some is needed so the consorts have a healthy balance between logic and emotion.
We don't know much about the weapons for Bards apart from Gamzee's clubs. Heart players tend to have sharp objects (katana, claws) that they hold in their hands. Since a Bard of Heart would start out acting like a Mind player, they might have a Mind-based weapon, something strategically concealed in plain sight like the sword in Terezi's cane.
Overall, an interesting classpect capable of some nifty abilities. Thanks for reading!