Masami Utoku is a central character in The Rolling Girls, serving as the Best of the Hiyoshi Town Propellers vigilante squad in the nation of Tokorozawa. Under the secret alias Maccha Green, she operates as a peacebroker-for-hire, defending her town from rival clans. She is the only daughter of the Utoku-en tea shop, which is located next door to the Moritomo household, and she shares a close childhood friendship with Nozomi Moritomo. Physically, Masami has long, messy brown hair, deep grey-blue eyes, and often wears fake reading glasses on top of her head.

Masami's personality is defined by a strong sense of duty and protectiveness. She acts as a dependable older-sister figure to Nozomi, encouraging her growth while also shielding her from danger. Her motivation stems from a desire to maintain peace in her region, a goal she pursues through direct combat as a Best. She wields a moonlight stone to enhance her abilities, but the repeated strain of battle eventually takes a heavy toll on her body. This culminates in a severe injury following a prolonged fight with her longtime rival, Kuniko Shigyou, an incident that renders Masami unable to continue fighting. Her injury becomes the catalyst for Nozomi's journey across Japan.

Masami's key relationships include her childhood bond with Nozomi, whom she admires and tries to support from the sidelines, and her rivalry with Kuniko, which has deep roots in their shared history as Bests. Despite her initial role as an active protector, Masami's development centers on learning to trust Nozomi and the younger generation to carry on the mission. She transitions from a frontline fighter to a recovering figure who passes the torch, allowing Nozomi to take up her requests and find her own path. Masami's notable abilities lie in her combat prowess as a Best, enhanced by her moonlight stone, and her iconic disguise as Maccha Green, which she uses to inspire and lead the Hiyoshicho Propellers.