Tomoe Udagawa, a third-year Haneoka Girls' High School student and drummer for Afterglow, stands 168 cm tall with long maroon hair and turquoise eyes, making her the band's tallest member. Her style features mature punk-inspired clothing with studs or chains, contrasting her younger years of shorter or ponytailed hair. She is the older sister of Ako Udagawa, Roselia's shortest member, and their inverted eye shapes and color schemes highlight their physical differences. Tomoe actively supports Ako's drumming journey as her primary inspiration, with Ako dubbing her the "world's coolest drummer." They share mutual admiration, gaming sessions, and the ritualistic "soi-ya" chant before performances. Tomoe treasures childhood memories with Ako and firmly declares no one else can "have" her as a sister. She observes Ako's growth into reliability, notably when Ako nursed her during an illness, reversing their caregiving roles.

Tomoe maintains a relaxed, candid demeanor, rarely speaking negatively or holding grudges. As Afterglow's stabilizing force, she prevents conflicts with a mature yet cool approach admired by peers. Despite her calm exterior, she exhibits a hotblooded streak when defending loved ones, occasionally backfiring. She uniquely connects with unconventional thinkers like Kokoro Tsurumaki and Kasumi Toyama. Her interests include taiko drumming—performed at local festivals for over a decade, earning community respect—alongside fashion, landscape photo book collecting, and her school's dance club. She works part-time at Galaxy Ramen with Masuki and previously at a fast-food restaurant with Himari Uehara. Popular at Haneoka alongside Kaoru Seta, she responds modestly to recognition and cultivates friendships with shopping district adults and close confidant Saaya Yamabuki.

Within Afterglow, Tomoe ensures cohesion through reliability and emotional support, regularly interacting with bandmates Ran Mitake, Moca Aoba, Himari Uehara, and Tsugumi Hazawa, plus associates like Yukina Minato and Sayo Hikawa. A notable moment involves her and Moca sharing an identical "When's the next performance?" voice line, inspiring a Halloween event where they humorously pretended to switch bodies after a theme park accident. Tomoe has no disliked foods, favors pork-and-soy-sauce ramen, and celebrates her April 15 birthday as an Aries. Her development includes recognizing Ako's increasing maturity while reflecting on her own role as a supportive sister, strengthening their bond through shared challenges.