YES!!! He is in a lot of Greek Mythological stories.
Hestia was not in any greek stories
No, he did not have any rivals in Greek mythology.
No, there were not.
In traditional Greek Mythology neither Hades nor Persephone have any children.
Hades is related in some fashion to every Greek God.
They were both important Greek gods.
Hades and Hercules had a complicated relationship; but Hades seemed to favor Hercules, for he allowed the taking of Perseus and Cerberus from his realm by Hades.
Hades and Persephone do not have any children.
Yes, both in Greek art and in modern.
No. Neither He nor His wife Persephone have any children.
For a Greek god, no, he was actually quite normal.
He was associated with both Orpheus and Hercules.