Sen has a tall, thin build with long, light-colored hair tied into a ponytail and framed with straight bangs and a long strand hanging from the right side of her face. Her eyes are light-colored and narrow.
Sen is a pretentious and perfectionist woman. She looks down upon people that she feels are lesser than her and is known to chastise them as well.
Sen participates in Stars Agency's Annual Audition and makes it past the fifth round, landing her into the top four alongside Yu Kamiya, Ai Momono, and later Kei Yonagi. During the final elimination round, she watches Kei wave to her younger siblings and notes that her lack of concentration would cost her the audition.
The actresses are given the scenario of a feral dog attack and Sen nervously contemplates how the group would be able to work together against something imaginary. However, Kei takes the lead by anchoring herself into a defensive stance and Sen follows along, relieved that she has experience with fight scenes. As Kei faces off against the dog, Sen and the other auditioners observe her method acting ability in surprise.
It is later revealed that Sen was the sole auditioner to pass, successfully becoming part of Stars Agency.
After making it into Stars, Sen is cast among Chiyoko and ten other actors from the agency to fulfill the roles of twelve out of twenty-four students for the Death Island movie. On the second day of shooting, she acts out a fight scene with Ryugo Donoe whom she berates for not being serious enough in his act. They later film their second take where Sen slashes Ryugo, making Kei throw up at the sight.
While the actors take a break for a meal, Ryugo and Sen chat about Kei's performance. Sen mulls over the fact she was accepted into Stars over Kei given the latter's abilities. However, Ryugo tells her off for her lack of confidence as an actress from Stars, ensuing an argument between them.
The actors later head to a cliffside to film another scene. Ryugo joins as a bystander so he can observe Kei and encourages Sen to not lose against her, to which Sen stammers that she won't. During the take, Sen acts out a character chasing Kei, Akane Yushima, and Kanna Kinashi towards the edge of the cliff next to a waterfall. Much to everyone's surprise, Kei's immersion in her role makes her jump off the cliff into the water. Not wanting to lose to her, Akane and Sen both follow suit. Ryugo and Yuri Kujo notice how the rivalry has made Sen different.
Director Yukiji Tezuka later talks to Sen, Kei, and Akane girls about their actions, expressing his contentment with the shot and his admiration for their fortitude. Ryugo interrupts the director and asserts his distaste for how far the three went, telling Sen off when she reminds him about how he told her to not lose against Kei. He continues on about how he didn't want her being influenced by an amatuer like Kei, ignoring Sen when she tries to get him to stop.
- Wakatsuki's favorite foods are manju, red bean jelly, and green tea.
- Her hobbies are running, bicycling, kickboxing and sword fighting.
- Her favorite movies are The Terminator, Nikita, Kill Bill and Million Dollar Baby.