Sae is a tall and beautiful woman with upper-back length, ashen-brown hair that is asymmetrically parted on her right side and brown eyes that are sometimes depicted with a hint of mauve red. She wears a black business suit with a black turtleneck. Her blazer's cuffs and the lower front have clamshell-like designs. Accessory-wise, she wears a necklace of a section sign (§) which references her judicial occupation (it also appears in the top right of Shujin Academy's emblem), earrings and a few dark hair clips can be seen above her right ear. She has violet fingernails and wears dark gray pantyhose with black high heels. She is sometimes seen with a smartphone and a black bag slung over her shoulder.
Shadow Sae has yellow eyes, extremely thick black eyeliner and black lipstick. She still wears her earrings, but she now has a black choker with spikes and a hat that resembles an American flapper from the 1920s, with playing cards stashed behind a golden rose. She wears a revealing black dress with purple laces which exposes the center of her torso and her right leg. Her legs are decorated with fishnet stockings, with the exposed right leg having a derringer pistol, and she wears purple high heels. She wears long black gloves. In addition, she also has tattoos on her shoulders and back which is a black dog's head wearing a choker surrounded by yellow roses.
While she does not initially transform into a demon form during her boss fight, she becomes Leviathan after the Phantom Thieves get past her cheating. Leviathan resembles a giant knight in black spiked plate armor, a helmet resembling a horned demon, several large firearms (such as a rocket launcher) mounted on her right hand and wielding a large red sword in her left hand.
Sae is intelligent, professional and serious, although her actual maturity is debatable. A workaholic, Sae is very strict in both her work as prosecutor and guardian for her younger sister, Makoto, constantly telling her to focus on her studies and such. With many responsibilities, Sae is always working due to Japan's work pressure, and is very stressed and exhausted. She must care for Makoto, almost putting her in a single mother's role. Her father dying in the line of duty and forcing her to raise Makoto acts as her very sore spot. She resents Makoto, as the Niijima sisters’ father throwing his life away in the name of justice forced her to raise her sister alone. The resentment is so horrible that when Makoto asks for her opinion on whether she would think the Phantom Thieves of Hearts were just, Sae snaps and says several cruel things that she instantly regrets, up to and including that she views Makoto as a burden, which hurts Makoto.
Because of her father's death, Sae views the concept of justice as meaningless. This has made her hyper-competitive and obsessed with getting ahead, to the point of telling Makoto that success is all that matters in life. This is to her detriment, as it results in many unjust prosecutions that she blatantly rigs in order to maintain the court's reputation, even if the people indicted by her are innocent.
The desire to frame at all costs also clouds her judgement and leads to her trusting all of the wrong people. Throughout the investigation of the Phantom Thieves, she remains completely unaware that her boss is taking her for a complete fool and while she doesn't get along with Goro Akechi, she nevertheless trusts his judgement regarding the Phantom Thieves and unaware that he is actually a double-edged sword. She also gets angry at every single NPC the protagonist makes a Confidant with because she suspects that they helped him commit crimes as a Phantom Thief, with many innocents that happen to befriend the protagonist such as Chihaya Mifune or Shinya Oda, or even people among her circles such as Makoto or Akechi being suspected as accomplices. The reasons for her suspicions are of a petty nature, such as Mifune giving him advice to evade the law or Yoshida teaching him how to make speechcraft for his calling cards. This also results in her more often than not getting into the way of innocents for the sake of getting a conclusion she wanted to present to her superiors, as she's easily willing to believe that Sojiro Sakura, the manager of a café she frequents, is abusing his daughter (though Makoto suspects the same thing for a time) and completely refuses to change her mind about the Phantom Thieves no matter what good deeds they do, to the point that she's willing to believe the worst of them at any time.
Sae's most consistent flaw is her hypocrisy. For all her supposed disgust of the Phantom Thieves, she's perfectly willing to resort to methods identical to corrupt official’s order to catch them, claiming that they "can't be caught by following the rules." Akechi even points out to her that, in doing so, her actions are no different from those of the very people she claims to hate. This trait of hers is especially prominent in her Shadow, who repeatedly states that she wishes to do things "fair and square" despite constantly cheating.
Envious of her sister and colleagues, Sae feels they have freedom to do whatever they want, while she is stuck in a dead-end attempt to defend the court's tarnished reputation. This envy, combined with her growing disillusionment and frustration with her profession as a concept due to being regularly strung along by political corruption, manifests as a Palace resembling a casino and a hyper-competitive and dishonest Shadow Selfhellbent on winning, desiring everyone to feel as helpless as she does while stuck in an unfair and corrupt system. She forces the Phantom Thieves into rigged gambling games that are impossible to win fairly. Her distorted and pitiful worldview is shown during her battle quotes, claiming that a society must have rules and order to operate, and those who do not follow society's rules must be punished and penalized but also that it is natural and right to alter the rules to favor one side, then call the rigged victory "justice." After being caught cheating in her supposedly "fair" game, this is her final straw as she goes berserk and attempts to kill everyone, offhandedly revealing that she didn't think it was possible to be fair until she gave up on the game.
Regardless of her many flaws, Sae isn't without standards or moments of compassion, a prominent example being when the protagonist was feeling fatigued over the drug injections making her feel concerned for him despite him being labeled a criminal. There were even times when she would be worried the protagonist would pass out. She is also genuinely disgusted and even disturbed at the cops for inflicting such pain on the protagonist. Despite Sae's hard demeanor and bitter relationship with Makoto, she loves her sister deeply and has even shown consolation for her, as it's indicated that some of her stricter attitude is born out of a desire to keep Makoto out of trouble and ensure that she succeeds.
Furthermore, unlike most other Palace rulers, Sae is open to reason. After defeat, the Phantom Thieves convince her Shadow to let go of her jealousy and resentment without stealing her Treasure. As a result, Sae has an epiphany about how pointless her grudges and overcompensation are without fundamentally changing her personality by using her feelings in a more proactive way, even going as far as forming a confidant herself and helping the protagonist trick Akechi and Shido so he won't get murdered.
