Behind the Opposito Senshi
The Opposito Senshi, who are the main enemies of the game Sailor Moon: Another Story, have the names of Sin, Nabu, Nergal, Marduk, and Ishtar. Their leader has the name of Apsu. Sin has a younger brother named Anshar - he has a pet called Kishal. These names were not chosen randomly out of a book of names, but have special symbological meaning to them, as does almost everything within the universe of Bishoujo Senshi Sailor Moon.
If you are familiar with the mythology behind Sailor Moon, then you already know that the Senshi have myths connected with them - the goddess Selene and Usagi, the Japanese tale of the Rabbit in the Moon, that the powers of Sailor Saturn and Pluto seem mixed up unless you look closely, and that Naoko Takeuchi, the creator of Sailor Moon, did not just stick to any one mythology - she drew from a rich multitude of myths. Information on these and all the other myths in Sailor Moon can be easily found on the web. What is not so easily found, at least from the websites that discuss the other Sailor Moon myths, are the myths behind the Opposito Senshi.
The names of the Opposito Senshi come mainly from Babylonian mythology. The following is the result of web-searches that I have done on their names. The websites that I used are listed - the main one that I trust the most is Pantheon.org, which Wikipedia was checked against at the time of this page's creation. All other sources are a result of a Google search.
Only after I was done writing this article did I find a far more concise one at Dies Gaudii.
Opposito
It can be inferred from the word "opposito" that it has the meaning somewhere along the lines of "opposite." The origin of the word is most probably Latin or Greek, as that is where the closest spellings of "opposito" occur.The theme of opposites among the senshi is prevalent throughout the game. There are five enemy senshi who are counterparts to the five senshi from the first season of Bishoujo Senshi Sailor Moon. They have powers similar to their counterparts, appear to battle their counterparts, and have somewhat of a like appearance. The opposites are Nabu and Mercury, Nergal and Mars, Ishtar and Venus, Marduk and Jupiter, Sin and Moon, and Anshar and Chibi Moon.
Later in the game, the Black Moon Sisters - Beruche, Cooan, Karaberas, & Petz - appear. They are opposite to the Sailors Mercury, Mars, Venus, and Jupiter. This is seen in their powers, their battles against the senshi, and have a little of like appearance. The opposites are Beruche and Mercury, Cooan and Mars, Karaberas and Venus, and Petz and Jupiter.
"AB OPPOSITO. From the opposed [point of view]." Found at TRADITIO Traditional Roman Catholic Internet Site.
"The opposito is another device whereby a fact is stated by denying its opposite." Found at Free MonkeyNotes - The Canterbury Tales by Geoffrey Chaucer.
Nabu
From Pantheon.org, Nabu is listed as an "ancient Sumero-Babylonian god of knowledge and writing, scribe of the gods. He is regarded as the son of Marduk."
In searching for a connection between Nabu and the planet Mercury, I did find that there is one, though disputed. It seems that while Nabu is associated with the planet Mercury, "...the planet [was] originally associated with the deified Nimrod..." according to AWP II.
Sometimes instead of Nabu, "Nebo" is written instead. It seems that "Nabu" and "Nebo" are either the same deity or different deities with the same function.
Nergal
From Pantheon.org, "Nergal is an ancient Sumero-Babylonian deity and the god of the netherworld..." and "...an evil god who brings war, pestilence, fever and devastation." Although Pantheon.org states his "...attributes are the club and the sickle," these two weapons never appear with Nergal in the game.
Searching for a planetary association for Nergal, Wikipedia asserts that "[i]n the astral-theological system Nergal becomes the planet Mars..." which has some interesting tie-ins with Marduk.
Marduk
Marduk is Sailor Jupiter's counterpart. The (male) diety Marduk is "a fertility god, but originally a god of thunderstorms" according to Pantheon.org, and he also "defeated Tiamat and Kingu, the dragons of chaos, and thereby gained supreme power." (Pantheon.org) Supreme power is often associated with the planet Jupiter - which is associated with Marduk (see below). Tiamat is a "huge, bloated female dragon" that was Apsu's consort.
Marduk is said to have cut Tiamat in two, creating the sky from the upper half and the earth from the lower, her water formed the clouds, and her tears became the source of the Tigris and Euphratus rivers.
The planetary association for Marduk is a little confusing. According to Nimrod, Mars, and The Marduk Connection, Marduk was originally associated with the planet Mars. He was also the "great god of war." The text shows that although Marduk originally was associated with Mars, it has changed since then, as Nergal is also associated with Mars.
According to Iraq Resource Information Site, Marduk was "quite specifically associated with the planet Jupiter."
Returning to AWP II, I found that the text concerning Nabu also contained some information on Marduk, stating "...Marduk’s ascendancy and correlation with Jupiter..." which supports Iraq Resource Information Site's statement.
So perhaps the real history of Marduk's planet was that he was originally the deity associated with Mars, but changed to Jupiter. This is strictly my own conclusion, as none of the text says so specifically.
Ishtar
To start with Pantheon.org again, "Ishtar was the ancient Sumero-Babylonian goddess of love and fertility." Many myths concerning her describe her as an evil woman who destroys her lovers. She also has a part in the epic of Gilgamesh. Interestingly, this is the first of the four Opposito Senshi to be named after a female deity.
The site Crystalinks Methaphysical and Science Website is a fair resource for information on Ishtar, where it is stated that Ishtar's "...emblems were the moon and the morning and evening stars (the planet Venus)."
At Assyria Part Sixteen, all four of the deities discussed by now are named with their planets - "The planet Venus was from the earliest times the star of Ishtar (and her Sumerian doubles of various names);the astrologers connected the other planets with the greater gods of the Babylonian pantheon, but with which gods they were severally associated, and whether at all periods the same gods, is still subject to controversy. For the Assyrian and Neobabylonian period it seems to be fairly made out that Jupiter was assigned to Marduk, Mercury to Nabu, Mars to Nergal, and Saturn to 'Nin-ib.'"
Sin
Sin is the "Sumerian Nanna"- the moon God - according to Pantheon.org, as well as the "lord of the calender, fixing the seasons, and also a vegetation deity and patron of fertility." Sin is also a male deity.
Anshar
Kishal
I found "Anshar" and "Kishar" at Wikipedia, where it names Anshar as a sky god, "...the husband of his sister Kishar" and Kishar "represents the earth as a counterpart to Anshar."
At Life Before and After - A Vision, it speaks of "Anshar (heaven) and Kishar (earth)" as being the children of serpents.
Apsu
Apsu is perhaps one of the weirdest 'deity,' especially since he is a "Sumero-Akkadian god who personifies the primordial abyss of sweet waters underneath the earth" according to Pantheon.org. According to a Sumerian myth, man was fashion from clay of Apsu.
At RealMagick, Apsu is a primordial dragon who is slain by a deity called Ea who takes up residence in Apsu's carcass.