Pluto

Pluto, also known as Dis Pater, was the Roman god of the Underworld.

Origin
Pluto was one of the six children of Saturn and Ops, alongside Jupiter, Neptune, Juno, Ceres and Vesta. All save for Jupiter were swallowed by their father, he forced Saturn to vomid the five other gods out and united they overthrew him.

Jupiter, Neptune and Pluto then drew straws on which aspect of the universe they would rule: Jupiter became the lord of the skies, Neptune lord of the seas and Pluto became lord of the Underworld.

Abduction of Proserpina
Due to being confined in the underworld and it's connection to the dead, Pluto's change to get a wife seemed unlikely.

He eventually abducted Proserpina, the daughter of Ceres. Ceres was upset of this, and as goddess of agriculture and fertility, refused to fulfil her duties, which led to massive droughts and famines in the realm of the humans. Jupiter, worried about humanity, contacted Pluto and begged him to release Proserpina. Pluto agrees, but it is also revealed that Proserpina had eaten a pomegranate, which confined her to the Underworld for half of the year.

The time of the year when Ceres and Proserpina would be together became spring and summer, while the time when she returned to the Underworld would be autumn and winter.

Later writings
With the rise of Christianity in the Roman and Greek world, many of the old polytheistic deities were viewed as demons, especially those who were tied to the Underworld. Pluto (or Dis) notably became conflated with the figure of Satan, and in many medieval and renaissance he is depicted as the ruler of Hell and enemy of both God and men. Notable such works The Divine Comedy, Belfagor arcidiavolo and Jerusalem Delivered.

Relatives

 * Terra - Grandmother
 * Caelus - Grandfather
 * Saturn - Father
 * Ops - Mother
 * Janus - Uncle
 * Venus - Aunt
 * Jupiter - Brother
 * Neptune - Brother
 * Juno - Sister
 * Ceres - Sister
 * Vesta - Sister
 * Proserpina - Niece/Wife
 * Mars - Nephew
 * Vulcan - Nephew
 * Bellona - Niece
 * Juventas - Niece
 * Apollo - Nephew
 * Diana - Niece
 * Minerva - Niece
 * Mercury - Nephew
 * Bacchus - Nephew
 * Triton - Nephew