Persona 5 Royal comes with insane amount of updates/additions to the base Persona 5 game. One of those additions is the new party member "Kasumi Yoshizawa." This article is about her and why I think she has one of the strongest character arcs in the game, in part to her ties to the conflict of the exclusive final arc of the Royal version.
Major Spoilers For Persona 5 Royal's New Content
Despite being completely absent in the base Persona 5, Kasumi is introduced in the first 10 minutes of the Royal version. The Phantom Thieves were in their final mission before Joker's arrest: The Casino Heist. Kasumi is introduced as a Persona user who isn't a part of the Phantom Thieves, yet she has a friendly relationship with Joker, enough to help him escape the Casino.
Her story and relationship with Joker is explored in short segments throughout the whole game year. While she doesn't affect the story of the base game, she appears at different times between the major arcs of the base story, and she finally gets her spotlight in the third Semester.
Kasumi: The Honor Student
At first glance, Kasumi seems the girl that got everything. She's a nice girl from a well-known family. She's a honor student who got a scholarship for being national level gymnastic.
A while before the events of the game, her younger sister has died in a car accident. Depressed, Kasumi who couldn't fulfill her promise of reaching the top of gymnastic world together with her sister anymore, couldn't achieve the results she was used to. Her family sent her to the counselor Dr. Maruki to help her with her depression, while she worked harder on her training to get back to her top form and do better at the meetings.
As a freshman in Shujin academy, she's an underclassmen to all the Phantom Thieves who attended Shujin. Joker first meets Kasumi when she tries giving her seat on the train to an old lady. He meets her again and saves her from someone who tried to pick on her in the city. They interacted more on the park clean-up activity sponsored by the school. That's when Joker first learns her name and they start hanging out.
Kasumi's Confidant is a very unique one. Initially there are only 5 stages instead of the usual 10. (She's the only character that has only 5 stars in the menu.) She's also unusually cheerful in those story segments but she gets depressed easily when she makes a mistake. In her Confidant story she teaches Joker about gymnastics and trains him for it. (The players HP increases along with her Confidant Rank.)
One of her Confidant interactions revolves around going to the Batting Cages with Joker. She misses all the balls, most of them without even swinging, while talking about how good she was as a child and claiming she's concentrating.
Kasumi also has an interesting opinion when asked about the Phantom Thieves. She thinks that they're doing the people harm by solving their problems for them. Believing that people will start depending on the Phantom Thieves instead of solving their problems themselves.
Scholarship Uncertainty
Few months later Kasumi had to represent the school in an important gymnastics meeting. Despite her efforts, Kasumi couldn't get back in her top form before the competition. She barely got the third place in meeting. Teachers at the school threatened her of retracting her scholarship if she didn't provide results. Noting that the third place isn't good enough.
Depressed, Kasumi took a stroll around town and found her way into a Palace in the cognitive world. Taking Joker and Morgana with her as they were nearby. She didn't understand what was happening around her or what the Shadows are, but she managed to awaken her Persona.
Joker and Morgana gave her a crash course on the Metaverse. She didn't join the Phantom Thieves however, only stayed friends with them instead.
After a while, she asked Joker to meet her at a certain date, but as he told her he has something to do that day she correctly concluded it was his Phantom Thieve job, so she went after the party and helped Joker escape, not knowing an ambush was waiting for him outside. (He gets better. As long you don't get a bad ending.)
Sumire: A Heartbreaking Backstory
Royal's exclusive third semester casts light upon the back story of the Yoshizawa twins and why "Kasumi" acts the way she does throughout the game's story.
After Joker's awakens in the "new reality" where everyone's wishes have been granted, Kasumi is the only other person aside from Akechi who noticed the weirdness and was able to see the Palace in the middle of the city that normal people didn't notice.
Exploring the Palace with Joker and Akechi, they stumbled upon a (cognitive) recording of the day Kasumi died. Upon watching that, this "Kasumi" broke down. Very hard!
You see, the real Kasumi Yoshizawa was the one who died before the events of game. The Kasumi we knew the whole time was her younger sister Sumire Yoshizawa. She couldn't deal with the death of her sister, (considering how she died, read below.) So she believed she's the one who died instead and lived her life believing she was her older sister.
Kasumi was a perfect gymnastic, full of confidence and cheerfulness and always got the top spots in the meetings. Her younger sister Sumire was the opposite, shy, hesitant and while she's not bad at gymnastics, she'd never get the top spot. Sumire grown jealous of her sister more and more. She viewed Kasumi's acts of help as a pity, and wished she'd just die so she won't drag her down. After all she was the inferior version of her in every way.
One day, both sisters were returning from school. Sumire was drowning in depressive and jealous thoughts. She didn't notice she was crossing the road at the wrong traffic light. Only when a car was about to hit her that she snapped, but as she was suicidal/depressed in the first place, she didn't try to dodge it. Her older sister Kasumi pushed her out of the way only to be hit instead, and she died just there.
Watching her sister die, Sumire's already broken self lost all will and reason to live. Eventually with the help of Dr. Maruki, she "managed" to block all memories of such incident. She began to live for her sister AS her sister! She got broken enough that in her cognition, everyone sees her as Kasumi even though in reality they still refer to her by her real name.
So when she finally remembered, she turned against Joker and Akechi shouting her anger about herself and pleading to be able to forget again. Fighting them to let them reconsider their stance on this "new reality" where all wishes come true.
A Reclaimed Life
It took some beating from Joker and a lot of time to cool down for Sumire to come back to her senses. She spent the next month with the Phantom Theives aiding in their quest to overcome the "new reality."
After that the second half of her Confidant story opens. Joker learns more about Sumire insecurities. Her vows to overcome them, and her love confession. By the end of her Confindat story, she finally manages to find her own style (of gymnastics) after imitating her sister style all these years.
My Favorite Character in Persona 5
So, "Kasumi" is my favorite character in Persona 5 with Futaba being a close second. (I'd also Romance Haru, but I have to admit that the latter doesn't have a good character arc.)
Her personality changed to match (her cognition of) Kasumi's but she still had the body of Sumire. That explains why she couldn't get back to her old form and win competitions. She wasn't even the same person. Sumire also retained some of her personality flaws that melded into her Kasumi persona. Her tendency to get depressed and her hesitation. Making her thinking she's acting unlike herself even more. Knowing that casts a new light on all of Joker's interactions with her. As well as the other people.
Early in Persona 5 Royal, students at Shujin were excited when the national level gymnastic attended the school. Only for them few months later to start whispering angry about her "honor student" status and that she doesn't deserve the schoolarship. Knowing her backstory we learn that they weren't angry that Kasumi was getting a special treatment without providing results. They were angry because a clearly unstable girl who pretends to be her dead sister is getting such treatment.
Another aspect I love about Sumire's character arc is how she believes she's inferior to her sister in every way. She isn't! For example, Sumire is good at cooking but Kasumi isn't. When she made a bento for Joker as Kasumi, it was very bad tasting. We don't see much of Kasumi's flaws, (that weren't actually Sumire's,) but I believe Sumire is better than her sister in more aspects than she's aware of.
Finally, I want to say that "Kasumi" is the closest love interest to Joker and the girl that has the most parallels with him. She's even the only one who confesses her love to him first out of the all (romance-able) party members. Some fans believe she's the Canon love-interest. And despite me choosing another girl as a romance partner, I would say she's the best girl in the Royal version of Persona 5.