Warmth
Hestia was the firstborn of the gods, first to be born by her mother, the titan Rhea, and first-eaten by her father, the titan Chronus. Chronus discovered that the gods, his children, would one day rise up against the titans, so to prevent this he decided, like any reasonable titan, to simply eat his children. Hestia went in first, then the rest of her sisters then brothers down the line: Demeter, Hera, Hades, Poseidon, . But the youngest, Zeus, was saved by Rhea, who had had quite enough of this eating her children business. Rhea sneakily put a boulder in place of zeus in his swaddling cloths, and the titanic babyeater gobbled up the stone in one swallow, oblivious to the trick. Then Rhea ferreted away the baby godling to an island with many goats herded by the goddess Almathea, and many bees kept by the goddess Melissa. It was a magical land of milk and money, and he grew strong there until he came back for his siblings and their revenge.
The titanomachy raged long and hard, but the gods were successful, and Zeus became king. Hestia was pleased by the end of the war. As the brothers decided which roles and responsibilities each of the gods were to carry, Hestia concerned herself with the construction of Olympus. She thought they needed a beautiful place to live and rule peacefully. She was deeply concerned with conveying a sense of balance, and warmth. So she built Olympus around a giant central hearth, with many comfy places to sit, good airflow, and many well ornamented corinthian columns.
Hestia's duties grew from that. Anyone who wished for warmth and order, for peace and beauty in their home gave her praise and sacrificed in her honor. First of the gods, most important to mortals, for without her, there was chaos.
But of course, men and gods beings as they are, they craved chaos too. By the time the twelve olympian ruling gods were in place, and the gods of defense and offense, Athena and Ares, were strong and evenly matched. Things were looking a little too peaceful.
Rising out of this peace came Dionysus. His mother was a mortal named Semele who Zeus had fallen in love with. Hera, the goddess of jealousy and wife of Zeus, convinced Semele that Zeus was hiding something by refusing to show her his true form. She begged of her lover to see his true form. That form was of course a bolt of lighting and when Zeus gave in to her demands she was struck dead, but not completely. Zeus called to his daughter Athena, wisest of gods, for a consult. It was said that Athena got her brilliance for having been born directly into Zeus' head, after Zeus ate her mother, for reasons. There she grew and when she was fully grown burst forth directly from her father's head. Athena saw that the charred remains of Semele included a baby godling, undestroyed by his father's true from. So together they fashioned a womb out of Zeus' thigh and for the second time, Zeus became a mother. When he was born for the second time, Hermes took him far away from the vengeful gaze of Hera, and he was forgotten by the gods.
But he wasn't forgotten for long. For he was the god of chaos and when he touched grapes he made wine. And when the gods touched wine, they lost their shit. They were so excited, and drunk, that they invited Dionysus up to party in Olympus, even though Olympus only had room for 12 gods.
Hestia was a little miffed. There was a big drunken party in her nice, peaceful home. But she was also the goddess of welcome, and she made space for Dionysus. Also, there was a certain attraction! She the goddess of order, he the god of chaos. well one thing led to another, and next thing you know it they're living together.
Olympus only had 12 thrones. But Hestia never wished to rule. She was content to make the house as orderly as could be made, with such a challenge now in residence.
Hestia's life was made easier when Dionysus departed. For he was a god of change, and could not remain in a single place, however majestic. And every time she welcomed him back, they discovered that each had learned how better to love each other.
And they lived happily ever after.