Sailor Mercury Development

Noriko Sakai, circa. 2000 - Takeuchi describes Ami as looking like Noriko Sakai.

Ami was not included in the original proposal for a hypothetical Codename: Sailor V anime, which instead featured Minako's very similar-looking best friend from that series, Hikaru Sorano. She was present, however, by the time the concept was expanded to center on Sailor Moon.
Creator Naoko Takeuchi designed Ami as the "team brain," giving her genius-level intelligence to create the impression that she was not quite human—in fact, the character was originally intended to be a cyborg with an accelerator. One possible storyline involved her losing an arm or being injured in some other way and dying from it, but Takeuchi's editor objected, so Ami became a fully human character.
Sailor Mercury's original costume design, like the others', was fully unique. It featured full-length sleeves, pink ribbons, shoulder guards, green accents, buttons on the stomach, and high-tech goggles. Later, Takeuchi was surprised by these sketches and stated that she did not remember drawing them. She also describes Ami as looking like Noriko Sakai, a J-pop idol of the early 1990s, and in Ami's original debut, Usagi thinks to herself that Ami resembles Miss Rain, a character from another Takeuchi series. This reference was removed in the 2003 renewal manga. In the English manga, a similar comment was made, but referencing Demi Moore.
Certain background details of Ami's character were chosen symbolically—for instance, her Western astrological sign is given as Virgo, which in astrology corresponds to the planet Mercury. In reference to a popular Japanese belief, her blood type is given as A, supposedly indicating sensibleness and fastidiousness.
The kanji in Ami's last name translate as "water" (水 mizu) and "field" or "civilian" (野 no); and her first name translates as "Asia" or "second" (亜 a) and "beauty" (美 mi). It is structured as a pun, as the syllable "no" indicates a possessive, so that her name can also be understood as "Ami of Water." It is frequently mistranslated as "Friend of Water" because of the French word ami, which is included in some Japanese dictionaries.
In the English manga and other books, "Amy" retains her family name of Mizuno. One early episode of the anime shows her answering the phone in her house by saying "Anderson residence," where in the original she had answered with "This is Mizuno," but this surname never appears again, and she is reverted to Mizuno by Cloverway in the series' third season.

Sailor Moon Usagi Tsukino Development

In the initial proposal for the Sailor Moon series, each of the five heroines had a completely unique outfit. Eventually it was determined that they would wear uniforms based on a single theme, and Sailor Moon's costume concept was the closest to that which would eventually be used for all the girls. The original did have some small differences, including color changes, an exposed midriff, and ribbons around the gloves and boots. She also had a mask, which would appear in a few chapters of the manga itself before being discarded. These aspects of Sailor Moon's costume are shown in multiple pieces of early artwork, along with a gun and cloak, which were also parts of the original concept.
Usagi's signature hairstyle derives from a "good luck charm" of the artist's; in college, Takeuchi would put her own hair up in odango before difficult classes or exams. In the initial sketches, Sailor Moon had pink hair. By the intermediate stages of development, Takeuchi planned to have the character's hair be blond in civilian form and change to silver when she transformed, but she was told by her editor that silver hair would be too plain for cover art. Nevertheless, stylistic use of differently-colored hair does sometimes appear in later artwork, and the concept of the heroines' hair changing color when transformed is used in Pretty Guardian Sailor Moon.
Of all the Sailor Senshi, Usagi's personality is closest to Takeuchi's own personality at the time Sailor Moon was created.
Certain background details of Usagi's character were chosen symbolically—for instance, her Western astrological sign is given as Cancer, which in astrology corresponds to the moon. In reference to a popular Japanese belief, her blood type is given as O, supposedly indicating friendliness, optimism, and carelessness.
The kanji of Usagi's surname translate as "moon" (月 tsuki) and "field" (野 no). Her given name is in hiragana usagi (うさぎ) and so its meaning is not inherent, but the word itself means "rabbit" and this is used as a pun frequently throughout the series, even in her hairstyle and possessions. Her entire name is structured as a pun, as the syllable "no" indicates a possessive, so that her name can also be understood as "Rabbit of the Moon." This derives from a Japanese folktale about the rabbit that is said to be visible in the Moon's face, much like the Western Man in the Moon. The English-language manga gives her the nickname Bunny to partially preserve this pun; many other localizations use the name Bunny as well. "Usagi" is not a common given name in Japan.
Similarly for the dubbed anime, "Serena" is probably at least in part a play on the word Selene, a moon goddess in Greek mythology who fell in love with a shepherd named Endymion. It also derives from the name of the series' moon princess, which is alternately either "Selenity" (from Selene) or "Serenity" (from the Sea of Serenity on the moon), both of which would be rendered in Japanese as [seɽeneti]. In an early DiC promotional tape that advertised Sailor Moon to television stations, Usagi was called Victoria.Another preliminary name that appeared on Kodansha's English website in an advertisement for the series was Celeste.