The moon kami is born in the most important birth sequence in Japanese myth, is handed the rulership of night, and then effectively leaves the corpus. One episode is attached to him, and the two chronicles cannot agree that it is his.
A name that is a job description
Tsukuyomi is transparently compounded from tsuki, moon, and a second element usually taken as yomi in the sense of counting or reading. That yields something like "moon-counting" — the moon as the instrument by which months are reckoned rather than as a face in the sky. An older reading, Tsukiyomi, survives in the names of the shrines. A minority of commentators have tried to connect the second element to the yomi of the land of the dead, which is written with different characters and is not the same word; the pun is tempting and the philology does not support it.
Some texts give him the fuller title Tsukuyomi-no-Mikoto and the Nihon Shoki variously calls him Tsukiyumi. The instability is normal for this period, when Japanese was being written down in Chinese characters by people improvising the system as they went.
The birth, and the division of the world
In the Kojiki he comes into being when Izanagi washes his right eye at the river mouth in Himuka, between the washing of the left eye that yields Amaterasu and the nose that yields Susanoo. He is given the realm of night. In the Nihon Shoki's main line the couple bear him deliberately as a partner to the sun, radiant enough to be sent up the pillar of heaven to rule beside her — a version in which the moon is the sun's colleague rather than her afterthought.
Then nothing. He takes no part in the cave crisis, has no children in the imperial line, quarrels with no one, and receives none of the elaborate epithets the chronicles hand out freely elsewhere. Given how much narrative energy the compilers spent on the other two, the silence is conspicuous enough that scholars have proposed the moon deity was originally more important and was written down, or that a separate lunar cult was folded into the imperial one without its stories.
The killing of the food deity
The one story runs like this in the Nihon Shoki. Amaterasu sends Tsukuyomi down to visit Uke Mochi, a deity of food. Uke Mochi prepares a banquet by turning her face towards the land and producing boiled rice from her mouth, towards the sea and producing fish, towards the mountains and producing game. Tsukuyomi is disgusted at being served food brought out of a body, calls it filthy, and kills her. When he returns and reports what he did, Amaterasu tells him he is a wicked deity and refuses to meet him face to face ever again. That is why, the text says, the sun and the moon are apart, one rising as the other sets.
From the dead deity's body the staples come: cattle and horses from the head, millet from the forehead, silkworms from the eyebrows, rice from the belly, wheat and beans from the genitals. Amaterasu takes them and establishes them as the basis of agriculture.
The Kojiki tells almost the same story with different actors. Its food deity is Ōgetsu-hime, its killer is Susanoo, and the occasion is his exile rather than an embassy. Both cannot be right. The most economical reading is that one killing existed in the tradition and each chronicle attached it to whichever brother its version of events made available — but which attachment came first is not recoverable, and it is the sharpest single example on this site of two near-contemporary official texts assigning the same crime to different defendants.
Shrines that do exist
Surveys of Japanese mythology usually list him as having no cult, which overstates it. Ise maintains a Tsukuyomi shrine as an auxiliary of the Inner Shrine and a Tsukiyomi shrine as an auxiliary of the Outer Shrine — two separate establishments with slightly different readings of the name. Tsukuyomi Shrine in the western hills of Kyoto is an auxiliary of Matsuo Taisha with a founding tradition reaching back to the seventh century, and small moon shrines exist across the country.
What he does not have is a festival calendar of his own or a body of petitionary practice. Japanese moon observance, when it comes, is the autumn moon-viewing of tsukimi — rice dumplings stacked on a tray, pampas grass in a vase, a full moon in the ninth lunar month — and that is a Heian aristocratic borrowing from Chinese practice, with poetry rather than the kami at its centre. The moon in Japanese culture belongs to the Man'yōshū and to Bashō far more than it belongs to Tsukuyomi.
Filling the gap
Modern fiction has found the emptiness irresistible, because a major deity with no personality is a free character slot. Naruto uses the name for an illusion technique and builds a whole cosmology on it; Ōkami's sequel, Persona, Nioh and a long line of mobile games make him a cold, elegant counterweight to a solar sibling, often gendered female. Anime routinely casts him as the aloof brother with the tragic secret.
None of this is derived from the sources, because there are no sources to derive it from. It is worth knowing when you meet a confident, detailed Tsukuyomi in fiction: the confidence is the author's, and the chronicles offer three sentences and a murder they may not even mean to pin on him.
Why is the moon deity male when so many traditions make the moon female?
Grammatical and mythological gender do not track each other reliably, and Japan is not unusual here — Mesopotamian Sin and Germanic Máni are male too, while Greek Selene and Roman Luna are female. What is unusual is that Japan pairs a female sun with a male moon, reversing the arrangement most familiar to European readers. That inversion is one of the reasons Amaterasu strikes newcomers as surprising, and it holds across the whole corpus rather than appearing as a local variant. Whether it reflects an old East Asian pattern or the particular needs of the imperial genealogy is argued both ways. The comparison of sun deities sets the pairings out.
If he rules the night, who rules the stars?
Effectively nobody, which is another oddity of this tradition. The chronicles name one star deity of consequence, Amatsu-Mikaboshi, the "august star of heaven", who appears as a recalcitrant kami who has to be subdued before the land can be handed over — a hostile star rather than a governed sky. There is no zodiac in the native material, no constellation catalogue, and no star myths comparable to the Greek ones. The star lore that Japanese readers do know, including the Tanabata story of the weaver and the herdsman meeting once a year across the Milky Way, arrived from China. A night sky ruled by an absentee deity with one crime to his name is, in that light, quite consistent.