It seems that at some time in your past your Heart was taken and consumed by darkness, transforming you into a Heartless - unlike most others, however, you’ve not only retained your original form, but also your intelligence and memories as well. While you only begin as strong as cannon-fodder Heartless like Shadows or Soldiers, you have plenty of room to grow stronger, and your other abilities as a being of darkness help compensate for your initial weakness.
To start, as a Heartless, your form has become, ironically enough, a Heart - what most other settings would call a soul - manifested into physical form of condensed darkness, granting you a high resistance to mundane weaponry, a nigh-immunity to dark-based powers, and, most importantly, a form of resurrective immortality: While magic and strong conventional attacks can damage or destroy you, this is only a temporary measure, as you will simply reform elsewhere given enough time, with only Keyblades and sufficiently strong ‘holy’ powers (such as purifying, unlocking/releasing, soul-manipulating, or extremely strong light-aligned magics) being able to put you down for good.
In addition, you also possess some other tricks beside that: A sort of sixth sense for the ‘darkness’ - in the sense of evil and negative emotions - within peoples’ Hearts, strong enough that true ‘villains’ and other vile sorts can be felt from entire worlds or dimensions away, though pinpointing will take experience; The ability to open ‘Corridors of Darkness’, shadowy portals that can lead you across worlds and other great distances without the need for something like a Gummi Ship - albeit inaccurately, and incapable of breaching into sufficiently ‘Light’ or ‘holy’ areas without great strength; And, lastly, the ability to steal the Hearts of others who are on the verge of either death or falling to Darkness, quite literally forcing it out of their bodies - and from there, consuming it and either slowly incorporating its strength into yours, or fully corrupting and shaping it into a new Heartless like yourself.
While there are more abilities to be discovered - like harnessing Tier Magic, or commanding Heartless of lesser power - those are less immediately available, requiring you to deepen both your skill and power before you can access them. You can also optionally decide to forsake your original form and take on an appearance similar to most other heartless.
(Note: If you are sufficiently strong-willed when taking this power, or otherwise falling to Darkness in the future, then your Nobody will also form alongside you somewhere in the world.)