Hyacinthus is Greek God and former demigod Son of Hades. Upon gaining immortality, he became the God of Flowers, First Bloom, and Rebirth. He is the immortal husband of Apollo, and the two are utterly devoted to one another. He was once human, but he was killed by Zephyros because the god loved him unrequitedly. Instead of dying, though, his lover Apollo brought him to Mount Olympus and--with Hades' blessing--made him his immortal spouse.
As the god of rebirth, Hyacinthus is the guardian of those souls who wish to be reborn to try for the Isles of the Blest. He is who they must appeal to in order to dip in the Lethe and be reborn.
Since then, Hyacinthus has acted as a bridge between Hades and Apollo. Hyacinthus holds a great deal of respect for his father, and he, therefore, transferred that respect to his husband. Apollo, having frequently met Hades outside of Olympian Council meetings through Hyacinthus, became the first Olympian and decades to actively support the Lord of the Dead, and that is largely due to Hyacinthus acting as a bridge between the two.