Kasumi and Sumire were twin sisters who were both very talented at gymnastics. But while Sumire was above-average, Kasumi was exceptional. Sumire got jealous of her sister's talents, and slowly developed depressive thoughts that she was a mere shadow of her sister. A month before the two were officially supposed to transfer to Shujin, Sumire ran away and was nearly hit by a car, but was saved at the last moment by Kasumi, who sacrificed herself to push Sumire out of the way.
Sumire's parents recommended that Sumire see a therapist named Dr. Maruki, who used his research into "cognitive psience" to help her deal with the Survivor's Guilt and trauma of her dead sister. Thus, Sumire became "Kasumi," and this new Kasumi thought that Sumire was the one who died. However, most people still see Kasumi as Sumire, and her physical abilities were not quite as good as those of the real Kasumi, threatening her scholarship as well as the possibility that her repressed memories might return and drive her to suicide.
Meeting with the Protagonist
Kasumi's first appearance in the main story of Persona 5 Royal is on April 12th, trying to lend her seat on the subway train to an old woman only for a man to suddenly take it from her. The Protagonist encourages her to stand up. Though Kasumi is too scared to do so, she thanks the Protagonist. At this point, her name has not been revealed.
Kasumi pops up at other points in the story later on, with her first real appearance on May 30 where she is accosted by a creepy man and saved by the Protagonist's intervention as they both head to a nearby park for a school field trip. The Protagonist learns her name is Kasumi Yoshizawa, and impressed by her gymnast skills and pleasant demeanor, the Protagonist decides to learn some gymnast skills from her.
Several months later, in October, Kasumi turns in a disappointing performance at a gym meet, with the school's Vice Principal threatening her scholarship if she does not get first place in the next big competition. Kasumi heads to the stadium construction site in Odaiba, and accidentally slips into a Palace with the Protagonist and Morgana close behind. She ends up running into a Shadow, but awakens to her Persona, Cendrillon. The three of them join forces to destroy the threat. Afterwards, Morgana offers Kasumi a spot with the Phantom Thieves, but Kasumi turns him down. Later in October, Kasumi has lunch with the Protagonist and Maruki, with Maruki remarking on her progress. She also dances on stage during a school festival, earning praise from her classmates.
In November, Kasumi appears again, reprising her role from the very beginning of the game as a playable character and helping Joker escape Sae's Palace. Kasumi then disappears until over a week later, when the Protagonist survives an assassination attempt in an interrogation room. When she hears the Phantom Thieves are about to take down Shido, she offers her help, but Morgana and The Protagonist dissuade her from joining, saying that Shido is too great a threat to involve her.
After the Phantom Thieves end the threat of Yaldabaoth, Kasumi meets with the Protagonist at Meiji Shrine on New Years Day. However, both of them notice that something is amiss, with the shrine practically empty, and Kasumi's father referring to someone else. The next day, Kasumi stumbles onto a Palace at the construction site in Odaiba in the real world. The Protagonist and Goro Akechi join forces with her to find the source of the recent oddities.
As the three travel through the Palace, Kasumi starts seeing reminders of her past, but the memories refer to her as Sumire. Fighting their way through this odd new Palace, the trio soon come face-to-face with its Ruler, Dr. Maruki, who admits to being behind the recent warping of their reality. Maruki then plays a clip from Kasumi's past, revealing that the "Kasumi" with the Protagonist this whole time was actually her twin sister Sumire, and the real Kasumi died in a car crash while saving her life.
Sumire loses her Phantom Thief form as her true identity emerges, and goes into shock as Maruki kidnaps her. One week later, Joker and Crow return and reject Maruki's offer. Sumire, suffering from dissociation, attacks Joker in her Phantom Thief form. When she is defeated, Maruki tries to forcefully "fix" her, turning her Persona, Cendrillon, berserk as she attacks Joker and Crow. However, the two are soon joined by the other Phantom Thieves who have broken free of their illusions and take down Cendrillon.
Afterwords, Sumire has trouble dealing with her remembered past as herself combined with taking on her sister Kasumi's identity for the past year. However, after overhearing the Phantom Thieves talking with Lavenza, she decides to join the party, and re-awakens to her Persona in Maruki's Palace. After helping the Phantom Thieves steal Maruki's Treasure and reverting the world back to normal, Sumire celebrates with the group. At the end of the game, she briefly meets with the Protagonist at the train platform before the two head their separate ways.
Kasumi appears as a playable party member a couple of times in the original story, but is not controllable. For the most part, though she is available as a Confidant under the "Faith" Arcana, Kasumi does not join the Protagonist's party. Furthermore, her Confidant ranking mysteriously ends at 5, while all other Confidants go to 10.
After the Protagonist visits Maruki's Palace twice in January, Kasumi joins the party as Sumire, with the Persona Cendrillon, and her Confidant ranking expands to include a max 10 stars. Like the other female Confidants, she can be chosen as a romantic option if the player so desires.
In combat, Sumire wields a rapier and a lever-action rifle. Cendrillon uses Bless element attacks, but also has some physical attacks and support skills that increase the party's chance of landing a Critical hit.