"If there's one thing that almost all of the Olympian gods agree on...it's to not pick a fight with Hades. And knowing how this guy is like, I find myself agreeing with them too."
Hades, also known as the Unseen One, is the king and god of the Underworld originating from the myths of ancient Greece. Born from the union of Cronus and Rhea, he later participated in the war against the Titans and successfully won along their siblings. While Zeus and Poseidon ruled the sky and the sea territory respectively, he claimed the darkest place, located inside the earth, where the light of the sun is not allowed to shine, which is the realm of the dead; although he's not the god of death himself, as the role would be taken by Thanatos.
His duty subsequently made him often unable to leave his own world and therefore, he is not considered to be one of the Twelve Olympians, but is still referred as one of the principal gods. He has many assistants and servants, among others are the judges who judge the human soul (Minos, Rhadamanthus, and Aiacos), Cerberus the guardian of the underworld, and the Moirai, who control over fate.
Hades fell in love with Persephone, daughter of Zeus and Demeter, and abducted her to live with him. When Hermes discovered this, he demanded Persephone be returned to Demeter and it was decided that if she had not eaten any food from Hades, she could return to the upper world. However, Hades tricked the girl into eating a pomegranate seed and therefore she could only return to the living for half of the year. This myth was symbolic perhaps of the cycle of life and death, planting and harvest.
Because of his status as the lord of the Underworld, Hades was greatly feared by the Greeks. So much so, that to merely utter his name would gain his attention, and even the Olympian gods would constantly be unnerved at the thought of Hades visiting their mountain when it came to matters concerning the balance. However, this means not that Hades is an evil deity, but in fact far from it. Hades was concerned with laws and order above almost all else and as such had a strong control over his temper with virtually no ego, especially compared to most of his siblings. Therefore, he is considered to be the most mature out of all his brothers and sisters. All Olympians were said to be unable to break a promises if sworn to it, but Hades was always honest, concerned mainly with remaining unbiased in light of the scope of his job.