Who were the twelve Greek gods and who were their Roman counterparts?

Zeus (Jupiter)
Poseidon (Neptune)
Hermes (Mercury)
Hera (Juno)
Hades
Dionysus (Bacchus)
Demeter (Ceres)
Apollo a.k.a. Phoebus (Apollo)
Ares (Mars)
Artemis (Diana)
Athena (Minerva)
Aphrodite (Venus)

Hades (Pluto)
Pallas Athene (Minerva)
Hephaestus (Vulcan)
Hestia (Vesta)

https://www.worldhistory.org/collection/58/the-12-olympian-gods/
https://www.britannica.com/list/12-greek-gods-and-goddesses

Should be more specific as there were far more than the list above. Eros, for instance, who's counterpart was Cupid. Surely he was of some significance. He was, after all, the son of Aphrodite, and was the God of Love.

The "Olympus Twelve" include Demeter (Ceres) and Dionysis (Bacchus), who don't make it on the list above, but instead we see Hades and Hestia. Hades, of course, didn't live on Olympus, but IS of significance, and Hestia was one of the original Olympian gods, but voluntarily relinguished her place to Dionysis. In return she was made the goddess of the Sacrificial Flame.

So even asking for the twelve "main" gods is misleading. The "Olympus Twelve" were to be worshiped as the most important twelve gods and goddesses, so as far as believers were concerned, they WERE the "main" gods.

Demeter (Ceres) was a greek goddess, but the top twelve are those that lived on Mount Olympus.

The Greeks developed a Pantheon of twelve major Gods, who lived in Olympus (which I have translated as Heaven). These were Zeus, Hera, Poseidon, Hades, Apollo, Artemis, Aphrodite, Ares, Athene, Hermes, Hephaestus and Hestia. Dionysus later took the place of Hestia. Uranus and Cronos were the previous generations of gods. The gods sometimes changed their functions. Originally there were different gods of the Sun and Moon, Helios and Selene, before Apollo and Artemis took them over.

The Romans had their own gods. Although they conquered Greece, they admired Greek culture, and they identified the Greek gods with their own gods. The Olympic Twelve became Jupiter, Juno, Neptune, Pluto, Apollo (the only one to stay unchanged), Diana, Venus, Mars, Minerva, Mercury, Vulcan and Vesta, with Bacchus as the late one.

https://www.desy.de/gna/interpedia/greek_myth/olympian.html
gwydir.demon.co.uk/jo/roman/teachers.htm no longer exists

At this website, Demeter is listed as one of the twelve:

https://www.britannica.com/list/12-greek-gods-and-goddesses

And at this one:

http://ancienthistory.about.com/od/romangods/a/022709RomanGrk.htm

And Bulfinch's Mythology has this to say of Demeter:
"One of the great Olympian deities of Greece"

Hades did not live on Mount Olympus, which was a fundamental part of being considered an Olympian god. He lived in the underworld. The three brothers, sons of Saturn and Rhea were Zeus, Hades and Poseidon. They divided Saturn's realm into three and each took control of one part. Zeus gained the heavens, and Poseidon and Hades drew for who would control the seas and who the underworld. Hades got the underworld.

According to Bulfinch's, the Olympians were the brothers and sisters Zeus, Demeter, Hera (also Zeus' wife), Poseidon, and Hestia (later replaced by Dionysis), and Zeus' children, Athena, Ares, Hephaestos, Phoebus Apollo, Artemis, Aphrodite and Hermes.

"The Larousse Encyclopedia of Mythology" (P. 97, Barnes & Noble edition) says: "First came the twelve great [Greek] gods and goddesses: Zeus, Poseidon, Hephaestus, Hermes, Ares and Apollo; Hera, Athene, Atemis, Hestia, Aphrodite and Demeter. Beside them were ranged other divinities, some of whom did not relinquish pride of place to the great twelve. Such were Helios, Selene, Dione, Dionysus, Themis and Eos...... It must be pointed out that Hades, although a brother of Zeus, did not frequent Olympus and, along with the goddesses Persephone and Hecate, remained in his subterranean empire."
I guess Poseidon left Amphitrite and Triton to look after things back at his place.