"Kasumi" is a seemingly cheerful, elegant and friendly person with a seemingly perfect life, as she received a Shujin scholarship and was nearly uncontested in gymnast competitions. Unlike most other members of the Phantom Thieves, her family is also fully functional and caring towards her well-being. She also appears to be quite clumsy and careless, as she is shown outright missing the mark at the batting cages even if her reflexes are usually excellent, and she lets her phone's battery run out while texting the protagonist. Despite the fact that she can cook, she is unpredictable and somewhat clumsy at it, since she isn't sure about the ingredients, and she often uses stamina replenishing food due to her gymnast tendencies. It's because of that she eats a lot, that she considers a "normal" amount for a gymnast like herself and she also has large lunch box to prove this.

Despite her supposedly cheerful and bold personality, she does feel depressed sometimes for no apparent reason, and when practicing gymnastics, she often felt like that her body isn't hers and she cannot execute complicated moves properly despite knowing how to use them.

While she expresses admiration towards the protagonist, she dislikes the very idea of the Phantom Thieves, as she believes that people should learn how to solve their own problems instead of relying on others. Although she acknowledges that the Phantom Thieves do good by helping people, she worries that people may become heavily reliant on them to solve their problems and stop making their own efforts.

Sumire herself is shown to be a lot more reserved and introverted person, in addition to being depressed and convinced of her own worthlessness to the point where she resents attempts to cheer her up, although this changes for the better as the protagonist advances her confidant. She suffered from an inferiority complex towards her near-perfect sister, fueled by being helped throughout her life by her. She has told the protagonist that Kasumi does most of the work for the twins bar preparing dishes, including arranging her very own bedroom. As a result, she has no self-confidence or self-esteem at all and delusionally believed that if she wasn't Kasumi, she should die, not acknowledging that Kasumi did not do that based on patronization but out of genuine care. The jealousy degraded into severe depression, which caused her to suddenly break down and try to run away from Kasumi without looking at incoming traffic, only for Kasumi to sacrifice herself to protect her sister from being ran over. She gained severe survivor's guilt as a result, even going as far as requesting Takuto Maruki to transform her into her older sister to artificially boost her non-existent self-confidence and ego, effectively putting her previous statements about the Phantom Thieves in irony as she was the very one who requested Maruki to solve her own problems instead of relying on herself.

It's implied that while Sumire was "Kasumi," the real Sumire still exists subconsciously, resulting in the sisters' strengths and weaknesses mixed up and colliding with each other, which contributes to her inconsistencies in gymnastics and cooking. This also explains her odd depressive fits when she was still "Kasumi," impairs her ability to perform and presumably led to her head teacher threatening revocation because the school had realized that she was impersonating a dead athlete. Other students also alienate her as they still see her as the inferior Sumire and not Kasumi, and for them, she is undeserving of the scholarship and is merely a mentally unstable person. (Although for her, due to the cognitive manipulation making her think that she's Kasumi, she just believed that people are jealous of her talent and special treatment.)

Despite Sumire's inferiority complex towards Kasumi, she is in no means, bad or even average at gymnastics, as she can often get into the high charts even if not consistently at 1st or 2nd during competitions, indicating that she is actually above average at it. People around her, including Kasumi herself had commented that Sumire, is in fact not lagging very far behind from Kasumi, she only underestimates herself to an unhealthy degree. Additionally, much unlike Kasumi, Sumire is said to be a decent cook, as when she was "Kasumi" she was able to prepare a curry bento with a similar taste as the curry served in LeBlanc.

Sumire also becomes easily flustered in pressuring situations, such as during the romance option in her Confidant, during which she stutters and freezes when the protagonist leans in to kiss her. Sumire's fashion senses are outright horrific and is prone to choosing bad or overly gaudy clothing.

In the Metaverse, Sumire is a significantly different person. She acts much more aggressively and determined, being a violent and focused fighter without any traces of her introverted tendencies in the real world. This indicates that much like the protagonist, Makoto or Haru, she suppresses her anger and can only relieve it in the Metaverse.