Mamoru Chiba is the civilian identity of Tuxedo Mask, a central figure in the Sailor Moon anime. He is the present-day reincarnation of Prince Endymion, the prince of Earth from the ancient Silver Millennium. Mamoru's early life was marked by tragedy: his parents died in a car crash on his sixth birthday. This event triggered memories of his past life and left him living alone. Throughout the series, he struggles with fragmented memories and a sense of lost identity. His primary motivation in the early story is to recover his past, which he believes can be achieved by finding the Legendary Silver Crystal.

Personality varies somewhat between adaptations. In the original 1990s anime, Mamoru is portrayed as academically gifted and generally outgoing, though he can be acerbic and initially argues frequently with Usagi Tsukino. He is shown as a college student with broad interests. In contrast, the manga portrays him as quiet, studious, mysterious, and stoic, admired by his high school peers for his cool demeanor. Across all versions, he is consistently depicted as intelligent, protective, and deeply devoted to those he cares about. He also suffers from loneliness and trauma due to the loss of his parents.

As Tuxedo Mask, Mamoru disguises himself in a formal black tuxedo with a top hat, cape, and white mask. His role in the story is to support the Sailor Guardians. He often appears at critical moments to offer advice, distract enemies with thrown roses, and physically aid in battle. Most of Sailor Moon's transformations and final victories are achieved through their cooperation. Notably, Mamoru is frequently kidnapped, brainwashed, or captured throughout the series, a narrative choice that keeps the focus on the female protagonists.

Tuxedo Mask possesses several notable abilities. He wields a Sailor Crystal that grants him guardianship over the planet Earth. He has psychic powers, including psychometry, which allows him to read the history of objects or people through touch, and healing abilities. These powers evolve and become more prominent in the manga. The manga and Sailor Moon Crystal also give him a named energy attack, Tuxedo La Smoking Bomber, a powerful blast from his hand. In the anime, his fighting style is more physical, using his cane as a weapon and throwing energy-charged roses as darts. He also demonstrates precognition through dreams that warn of danger.

The central relationship in Mamoru's life is with Usagi Tsukino, who is the reincarnation of Princess Serenity. They share romantic feelings well before discovering their true identities, and once they become a couple, they remain deeply devoted to each other. Their love endures many trials, including separations, brainwashing, and memory loss. In the future, they marry and become Neo-Queen Serenity and King Endymion, ruling over Crystal Tokyo. They have a daughter named Small Lady, known as Chibiusa. Mamoru also has a brief, ambiguous relationship with Rei Hino in the anime, which is not present in the manga.

Mamoru undergoes significant development. He begins as a character searching for his own past and purpose, uncertain of his true nature. He later worries about whether he hinders the Sailor Guardians rather than helps them. Over time, he grows into a confident partner and protector, fully embracing his role as Earth's guardian and Usagi's support. In the Sailor Stars arc of the original anime, he is killed offscreen by Sailor Galaxia but is eventually revived at the end of the series.