Everyone agrees that there are twelve Olympians, but the identities of the twelve seems to vary. What is the deal?
The shift has to do with Hestia originally being one of the twelve; then when Dionysus became a god, she gave up her throne for Dionysus. Interestingly enough, this unbalanced the council; there were then 7 men and 5 women. These were
- (initially) Hestia (then later) Dionysus
These specifically were the main twelve for several reasons:
A couple were direct children of Kronos, namely Zeus, Poseidon, Hera, and Demeter (and Hestia, but she gave up her seat for Dionysus; and not Hades, see below). This puts five on their seats fairly clearly.
The remaining gods were major in that they had governance over important things - Ares, god of war, Athena, goddess of war and wisdom, Hephaestus, god of blacksmithing, clever workmanship, and fire, Apollo, god of poetry, archery, sickness and healing, prophecy, and the sun (which he sometimes gave up to Helios, the Titan), Artemis, goddess of hunting, the moon (sometimes given up to Selene, the Titan), virginity/young women, children, and also archery, Dionysus, god of wine (wine was a lot more important back then because water was not safe to drink in most cases; therefore wine was more commonly consumed as the alcohol killed the germs/bacteria/gross stuff), Hermes, god of messengers, travelers, thievery, games of chance, cattle, and all that good stuff, and Aphrodite, goddess of love.
No one put minor gods and goddesses on the council because they just weren't important enough. Could you imagine have the god of bee-keeping and cheese-making on the council (Aristaeus)?
And then there was Hades. The reason he (and Persephone his wife) weren't on the council was because they were kind of scary, and he just didn't visit Olympus much. He had his own realm to handle (the Underworld).
There are many lists of the Twelve Olympians, and one can make an argument that any of the gods should or shouldn't be included. One of the most common lists is Zeus, Hera, Apollo, Artemis, Ares, Hermes, Poseidon, Demeter, Hephaestus, Aphrodite, Athena and Dionysus, and the Hestia gave up her seat in the Olympian throne room to Dionysus. This is the most popular list of the Twelve Olympians and is the one that is generally accepted by people who don't care too much. Although, really, why does it matter? There is no reason to group the Greek Gods by who had a throne on Mt. Olympus. I would argue Hades is much more important and powerful than Dionysus or Hephaestus. If we're grouping them by power level, Eros should really be on the list, considering many texts state that it is said that he is an entity feared by Zeus himself.
Tl;dr Usually the most common list has Hestia giving up her seat for Dionysus, but there is really no reason to group the Greek Gods this way. Arguments can be made for anyone's inclusion or exclusion on the list.
The Twelve great gods of the Greeks were known as the Olympians. Together they presided over every aspect of human life. The goddess Hestia (listed here in the second rank) was sometimes included amongst the Twelve.
So going with the list provided
- Hestia (Part-time)
They seem to vary because of the age of the sources, I believe earlier sources say that Hestia is one of the 12.
The 12 Olympians were:
Hestia / (later Dionysus): Hestia gave up her throne to Dionysus for a place next to the fireplace. https://greekmythology.wikia.org/wiki/Thrones_of_the_Gods
It is considered at many places that Hades is also one of the Olympians as he is one of the main off springs of Kronos and Rhea who helped Zeus defeat Kronos and throw the other Titans in Tartarus, however, he does not have a seat at the Mount Olympus as he needs to stay in the underworld in order to rule it.