Can Japanese/non-Japanese have a mononym or middle/multiple given names?

Magician "Teller" (on the right) legally
changed his name to a mononym last name

A mononym is a person with only one name: it may be a "family" name, a "given" name, neither, or a combination of both.

In general, non-Japanese foreign residents of Japan can have mononyms and middle names, hyphenated names, and separate last names for those who are married, with the pragmatic restriction that the name must be transliterated into alphabet and be identical to the alphabet name on your passport.

Japan foreign residents can also have 通称名 {tsūshō-mei} (registered Japanese kanji/kana aliases), and this name may also be a mononym or multiple names — good luck getting a mononym or three or more names recognized and recorded on most Japanese paperwork requiring a name though. However, with the new immigration laws, this name is not imprinted on the 在留カード {zairyū kādo} — it was with the old ARC (Alien Registration Card; 外国人登録証明書 {gaikokujin tōroku shōmeisho}) system. It is instead on the 住民票 {jūminhyō} (local Residency record) and the corresponding 住基カード {jūki kādo} (Residency Record identification card) if one chooses to get one.

Individual businesses may insist on making your name confirm either to the アルファベット {arufabetto} name on your password/Residence Card or force you to adapt your legal Japanese alias to Japanese paperwork for the purpose of the family/given two name system.

Those whose naturalize or are natural born have less options

Japan requires its nationals/citizens to have exactly two names: a family name and a given name written in either 仮名 {kana} (Japanese syllabet) or 漢字 {kanji} (Japanese sinograms), with no punctuation of spaces.

Some people who naturalize simulate a hyphenated name or multiple names be stringing them together in the family name field or the given name field.

"Zeta" is actually her separate middle name

For example, Catherine Zeta Jones is usually transliterated into Japanese as:

キャサリン・ゼタ＝ジョーンズ {Kyasarin·ZETA=JŌNZU}

If she were to legally naturalize and keep her current name in 片仮名 {katakana} form, her name would look like this:

ゼタジョーンズキャサリン {ZETAJŌNZUKyasarin}

The 戸籍 {koseki} (Japanese family register) itself separates the family name from each given name in the family unit. It does not allow for punctuation or spaces, but in day-to-day life, she and others in her situation where you can't tell where one word ends and another begins and which go together, one would add the 中黒 {nakaguro} (middle dot 【・】) to separate names, and use the 二重横線 {nijū ōsen} (double horizontal lines 【＝】) to show different names that are linked together.

Technically speaking, however, punctuation and spaces are never part of an official legal Japanese name.

So if you're from a country that only has one name and you decide to naturalize, what do you do? You're actually in a similar situation to most Japanese from before just before the introduction of the modern 戸籍 {koseki} in 1872, after the Meiji Restoration (明治維新 {Meiji Ishin}) in 1868 and just before the Meji Constitution (大日本帝国憲法 {Dai-Nippon Teikoku kenpō}) in 1890:

The Old Era: Japanese peasants/merchants had mononyms; family names only for samurai and nobility

The Japanese were not influenced by
Matthew Calbraith Perry's middle name

Until the late 19th century, having a family name in Japan was a privilege for the higher classes within the feudal society. Then along came Commodore Perry in 1852, the Meiji Revolution, the Meiji Constitution. Family registers for Japan existed before this time, but it wasn't until Meiji-style family registers when every Japanese subject was equalized and everybody's name (merchants, peasants, famers, and fisherman alike) got normalized.

So with the fall of the feudal caste system and the rise of the 戸籍 {koseki} (family register): commoners (平民 {heimin}) got family names.

Because nobody had family names prior to this, they just got made up. By late nineteenth century, Japan's population was large enough that this meant, compared to its neighbors, a relatively enormous amount of family names got invented out of thin air.

Most of the new invented family names were references to geographical features where the family was from, and Japanese names to this day tend to reference nature more than names from other cultures that use sinogram-based names.

CJKV sinogram (汉字 {hànzì}; 漢字 {kanji}; 한자 {hanja}; 𡨸漢 {chữ Hán}) based "family" names

language/culture	family names	comments
Vietnamese	~100	14 names are 90% of the people 1 name (Nguyễn: 阮) is ~40% of the people multiple given, middle & family names are possible
Korean	~250	3 names are 50% of the population
Chinese	>4,000	20 names are 50% of the 1B+ population 100 names are 80% of 1B+ population
Japanese	>100,000	most created at will after 19th century; most created when population had grown to a large (for the era) 30M people, reducing name extinction

The modern era: Only the Imperial Family has mononyms

Masako OWADA⇒Princess Masako
Michiko SHŌDA⇒Empress Michiko

In Japan today, the only Japanese who have only one name (no family name) is the Imperial Family. Even then, however, they don't use their names often. Even amongst themselves, they respect titles. The Empress addresses her husband, the Emperor, in Japanese as 御陛下 {go-heika} ("Your Majesty"), not 貴方 {anata} (you), 秋仁 {Akihito}, or あきちゃん {Aki-chan}.

The Emperor is unique in that in addition to many exclusive titles and pronouns he has to be addressed by, he has a posthumous name, which is the current era name on the Japanese calendar. So when Emperor Akihito passes, he will be known as Emperor Heisei (平成天皇 {Heisei Tennō}), just as his father, Emperor Hirohito, is now known as Emperor Shōwa (昭和天皇 {Shōwa Tennō}).

Q: How do they write this one name on a 戸籍 {koseki} (Japanese family register) when it is oriented today so that everybody is organized underneath one family name?

A: The royal family does not have 戸籍 {koseki} (family registers) like common Japanese nationals. When a "commoner" marries into the family, such as 小和田雅子様 {OWADA Masako-sama}, they lose their family name and only have a given name.

Q: If they have no family register (or residency record), how do they do things that ordinary Japanese nationals do, like vote or get a passport?

A: Japanese royals do not vote or participate in politics. Or express partisan political opinions. Especially with respect to the Emperor, the constitutional monarchy is strictly separated from have any political power. Recognized Heads/symbols/royalty of State do not need/use passports to internationally travel.

How modern passports handle mononyms and middle/multiple names

One legal name. No "family" name.

In countries where mononyms are common (for example, Indonesia) the printed page of the passport will often contain a single field: FULL NAME, rather than two split fields: SURNAME and GIVEN NAME(S).

Inspection officers at border controls rarely input one's data from the passport manually; they swipe or scan the MRP (Machine-readable passport) and the computer reads the OCR-friendly font in the MRZ (machine readable zone) to get all the information off of the first page. The first line of the MRZ will look like this:

P<XXXPRIMARY<ID<<SECONDARY<ID<<<<<<<<<<<<<<<

1. The first character, "P", means it's a passport, as opposed to a MRZ for another type of international identification
2. The second character is the type of passport if the country has more than one type of passport. For normal passports, this is indicated as blank/null with "<".
3. The "XXX" is the 3-letter country code ISO-3166 indicating what country issued the passport (ex. "JPN" = Japan, "USA" = United States, etc). This is not necessarily the person's nationality.

4. 

   The MRZ contains the simple 26 letter name

   The next field is the "Primary Identifier". For Japanese and many other people, this is the family name(s)/surname(s) converted into alphabet. Letters not in the 26-letter alphabet are converted. (ex.: "ß"→"SS", "Æ"→"AE", "Ö"→"OE"). All punctuation, is converted into "<". This is important because some passports may not use the simple 26-letter ABC Latin alphabet for the human readable name field(s). An example would be a modern Icelandic passport.
5. The next field in the primary/secondary separator. This will be two less-than signs: "<<".
6. The next field is the "Secondary Identifier", For Japanese and many other people, this is the "Given Name". For Americans and other westerners, this will be the "First Name" and the "Middle Name" or "Middle Initial" is there's no space. If the person has a true mononym as recognized by the passport ICAO 9303 specification, the Secondary Identifier will not be present.
7. The line will be padded out to the right to the 44th column with "less-than" ("<") characters.

Even though the international machine-readable passport specification supports it, countries such as Indonesia will often separate the first two words of a given name with "<<" rather than "<", making the first part of the Javanese etc. mononym the "family" name in the eyes of immigration databases and the remaining parts the "given name(s)".

An example of a complicated American name and its encoding in the passport MRZ:

George Michael Richards-Stevens, Jr.

Blue: Given "First" Name

Cyan: Given "Middle" Name

Lime: Family "Last" Name

Red: Name Suffix(es)

P<USARICHARDS<STEVENS<JR<<GEORGE<MICHAEL<<<<

Purple: "Primary Identifier"

Magenta: "Secondary Identifier"

How Residence Cards for Foreign Residents of Japan Handle Names

If she was legally Chinese/Taiwanese there
there might be kanji under the passport name

Because Japan's identification cards for non-Japanese need to handle all the various systems of names in the world (different alphabets, different name orders, one to many names) yet be able to be processed by Japanese bureaucracy and forms and computers that are able to handle only Japanese and the simple alphabet, the cards represent names by:

• Only using one field, "NAME" to handle all the names (one for a Javanese mononym to multiple for a Brazilian — they will run the name onto the reverse side of the card if necessary!) a non-Japanese might have.
• Only using the 26-character ABC アルファベット {arufabetto} (alphabet) to render names. They do this by using the passport name(s) in the passport order, by consulting the original fields if they are alphabetic and simple or using the transliteration provided in the passport's MRZ.
• If the passport has an official printed Chinese 漢字 {hànzì} on its front page (ex. a Taiwanese passport or a PRC passport) and the same or equivalent identical or corresponding sinograms are in Japanese, that name may also be present. Because it is non-trivial for a Japanese person to input non-Japanese sinograms, they will often use the passport alphabetic transliteration of the name for practicality.

How does the United States (and many computer forms) handle mononyms?

"My Virginia driver's license
says 'Pocahontas NLN'"

Considering that the United States is mostly a land of immigrants, and that many of the original Native American ("Indian") tribes had mononyms, you would think that the United States would be well adapted to handle just one name (or four or more). Most federal government and state governments, however, insist on two or more names.

For those Americans or non-Americans that only have one name, a placeholder is used to indicate that a name field was not left blank on purpose:

NFN

No First Name: No given names exist

NLN

No Last Name: No family name exists

Police and medical records may also use the terms "FNU" and "LNU" to indicate respectively, the first or last name is unknown (but may exist).