Sailor Moon's laid-back, bubbly personality, and the large cast of characters surrounding her, create stiff competition for anyone hoping to be Usagi Tsukino's best friend. The four Inner Guardians, Ami, Rei, Makoto, and Minako, all have their own special connection with Usagi. Her classmate, and the enemy's frequent target, Naru, also appears throughout the series. In the final arc, Usagi even gets close to Seya, who fills the void left by her star-crossed lover, Mamoru. However, the fraught relationship between Sailor Moon and Sailor Mars is the strongest and closest of them all, and essential to the overall plot.

Sailor Moon depicts Usagi and Rei’s friendship differently between its various adaptations. In the manga and its faithful adaptation, Sailor Moon Crystal, Usagi and Rei are close friends, but with little friction between the two. But in the 1992 anime and trilogy of films, Rei Hino is more aggressive and antagonistic towards Usagi, often deflating one another's egos and providing comedic relief absent from Crystal. Although their rivalry ranges from friendly to bitter, Usagi and Rei’s friendship is a consistent emotional focal point and raises the stakes of even the smallest conflict.

Sailor Moon and Sailor Mars Fight Like Sisters

A Bond Strong Enough to Withstand Constant Bickering

Usagi and Rei are often at one another's throats, which doesn't seem friendly, but anyone with a sister knows that this type of bickering is a very intimate form of communication. Usagi and Rei are comfortable expressing anger, jealousy, and disappointment with one another, which can be difficult emotions to approach with people outside the family. Their relationship mirrors the way Usagi and her brother, Shingo, interact, with teasing and, at times, outright disrespect. As biting as they can be, they always rise above and put their differences aside, and emerge stronger than ever.

The Guardian of Mars also serves as a romantic rival for the Moon Princess early on in the series, fighting for Mamoru's attention as Rei Hino, and Tuxedo Mask's as Sailor Mars. From their first meeting in season 1, episode #11, "Usagi vs. Rei: Nightmare in Dreamland," Rei and Mamoru hit it off, bonding over their mutual annoyance at Usagi's childish behavior. Rei gets closer to Mamoru in the following episodes but makes sure to taunt Usagi when she's with him, signaling that Rei's motivations are more about competition than romance.

Rei Sacrifices Herself for Usagi on Multiple Occasions

Rei's Death Is More Than the "Women in Refrigerators" Trope

From their budding rivalry at Dreamland to Sailor Moon cradling Sailor Mars' dead body at the end of Sailor Stars, Rei and Usagi's friendship is one of the most consistent dramatic beats of Sailor Moon. Such a strong emotional bond between friends motivates Usagi to continue fighting, but usually at the cost of her friends' lives. In the penultimate episode of the first season, each Sailor Guardian sacrifices themselves one by one to allow Sailor Moon to continue towards Queen Beryl's castle. Rei's final words to Usagi are a reassuring ission that she had fun arguing with her all the time.

These sacrifices keep the unserious and carefree Usagi grounded and committed in the face of danger. Sailor Moon is the least experienced Guardian, due to her high status as Princess Serenity in her previous life. Rei recognizes Usagi's inexperience, but believes in her potential, and gives Usagi the tough love she needs to be successful. In season 5, episode 30, "Countdown to Destruction: The Sailor Guardians' Last Battle," Eternal Sailor Moon holds a dying Sailor Mars' in her arms, whose last words are, "Don't be such a baby, I know you're better than that, Usagi."

Usagi Is at Her Funniest When Fighting with Rei

Sailor Moon Needs Humor to Balance the Sorrow

While dramatic, tearful sacrifices happen now and then, the 90s Sailor Moon uses Usagi's relationship with Rei to elicit humor and bring tension to the group. The Inner Guardians don't always agree when it comes to investigating and engaging the enemy, with Rei and Usagi typically taking opposite viewpoints. Many of the series' funniest moments come from Usagi's petulant, naive behavior, bristling against Rei's pragmatic and suspicious nature. Embracing comedy helps provide balance to Sailor Moon, allowing heartfelt moments to contrast with the sometimes heartbreaking nature of being a Sailor Guardian.

Humor is severely lacking from the reboot series, Sailor Moon Crystal, which adheres closer to the manga and its whimsical, romantic vibe. Usagi cracks a rare joke at Rei's expense in Sailor Moon Cosmos, about finally having a school uniform that matches her friends, except for Rei, who still attends a Catholic school. Otherwise, the girls are less aggressive and combative, and Usagi ires Rei for how graceful she is, rather than being jealous of her. This small change drastically alters the group dynamic, and Sailor Moon Crystal ultimately suffers from the lack of friendly tension between Usagi and Rei.