Zodiac signs have been used to predict personality traits for millennia, with the first known astrology guides being over 2,000 years old. When writing the legendary Sailor Moon manga, Naoko Takeuchi drew inspiration from the field to create her space-themed guardians. Nowhere is this more obvious than with Usagi Tsukino, whose zodiac sign almost perfectly reflects her personality.
Usagi was born on June 30, a fact found in both the manga's character profiles and the narrative itself, as she proudly announces it to her friends to remind them to get her presents. According to the zodiac calendar, this makes Usagi a Cancer. In astrology, the Moon has dominion over the zodiac sign of Cancer, meaning it's the planet that most strongly affects those born under the sign. This is very fitting as Usagi transforms into Sailor Moon.
Cancer is represented by the crab, and while Usagi doesn't have any nautical links, crab physiology is an interesting metaphor for the Usagi/Sailor Moon dichotomy. Sailor Moon is the strong exterior that fights and struggles, while Usagi is the soft underside that, while physically weaker, is crucial to the whole being's functioning.
Cancers are said to be highly emotional and empathetic, as shown in Sailor Moon's first episode when Usagi gets upset after she fails a test. Despite being late, however, she also stops to help Luna -- a highly overt display of her empathy. This theme of emotion continues throughout the series, with Usagi regularly having emotional outbursts and her foes often using her emotions against her.
Usagi's empathy is also displayed by how often she chastises her enemies for using people's emotions as a weapon, like when she tells Murido she'll pay for taking advantage of the dreams of pure-hearted girls. This is also shown by the times she tries to save her foes -- perhaps most clearly when anime Usagi offers forgiveness to Queen Nehelenia, an enemy who went out of her way to make her fight with Usagi extremely personal.
A Cancer's empathy can often blossom into full-on nurturing and maternal tendencies, linking to the moon's status as the planet of motherhood. This is shown when Usagi tries to look after Chibiusa, despite not understanding the finer points of child-rearing. While the pair do retain a rivalry throughout the series, it is clear Usagi wants what's best for Chibiusa and aims to help her overcome her problems. Doing her best to purify or forgive her foes is also an extension of this, as trying to help people get back on the right track and live their best life is a very motherly thing to do.
It is said that Cancers befriend others quickly due to their emotional honesty and intense loyalty. This perfectly describes Usagi, who makes fast friends wherever she goes. She is also extremely loyal throughout Sailor Moon, and often lays down her life for her loved ones. The first action Usagi performs as Sailor Moon is the perfect example -- while she is doubtful about Luna's story, the second she hears Naru's cry for help, Usagi runs to help her friend. In fact, she regularly faces her fears to save people she cares about, as seen by the terror in her eyes when she faces down Queen Beryl at the end of the story's first arc.
Usagi also shows one of Cancer's most substantial weaknesses: suspicion. However, unlike many forms of this weakness, her mistrust is very focused and tends to manifest inwardly as self-doubt and self-loathing. Externally, it appears through Usagi's bouts of jealousy toward the other soldiers and Mamoru, as seen when she becomes upset that Chibiusa is receiving more attention than she is. However, the internal suspicion is best shown by how often her foes, including the Dark Kingdom, brainwash Usagi's friends to prey on her fear of losing her loved ones due to her failures.
It is clear Naoko Takeuchi spent a lot of time ensuring Usagi's personality fits her zodiac sign, as she is basically a textbook Cancer. From a thematic standpoint, this makes perfect sense. Sailor Moon and its lore are heavily tied to astrology, the zodiac and the many myths inspired by it, so naturally the characters reflect this inspiration effectively.
