Sae Kashiwagi

Sae Kashiwagi (柏木 さえ Kashiwagi Sae) is the main antagonist who does everything to ruin Momo Adachi's life. Sae's fair features such as her pale skin and dark hair serve as a contrast for Momo; her "innocent" appearance belies her scheming, mischievous, trouble making nature. She is the one who spreads malicious gossip about Momo; something Momo knows, but has trouble dealing with, as people are more apt to believe in Sae than the 'airheaded ganguro girl' to the point that Kazuya "Toji" Toujigamori believed Sae over Momo (who was currently his girlfriend). She often uses the gossip to make people dislike Momo, as a result of her own jealousy and insecurity. She spreads rumors that she is dating Toji, and that Momo is trying to steal him from her when it is actually the other way around. She really is mean and doesn't care for Momo and isn't her "real" friend. All she wants is popularity in school, to humiliate Momo, and to get Toji all for herself. She will stop at nothing to complete this task. Later, she dates a male model named Jigoro and in the end exploits him to help break up Toji and Momo, the nastiest scheme she has ever plotted. She begins to repent after she falls in love with Ryo Okayasu and understands the hurtful feeling that she caused Momo, when Ryo seemingly gets her pregnant (it later turns out to be a medical condition that simulates pregnancy) and subsequently abandons her.

Peach Girl: Sae's Story
There is also a section in volume 1 which retells half of Peach Girl's plot from Sae's point of view where we learn that Sae is quite insecure and feels that friends and boyfriends do not come easy to her like they do others, which is partly the reason for why she is the way she is.

Sae later purposely injures the dog in order to keep it away. Sae starts modeling and pretends she goes to college with Momo Adachi, when she is really still in high school. She meets Honda and works for CC as a model where she meets a college model named Takuma who she uses to keep away Kanji. Another model is in love with him though and despite her efforts to win him over his indecisive nature causes them to break up. Later, she meets another boy named Shinji in an accident where she knocks Kanji and his bike into his car and offers to pay damages. She works hard in odd jobs to pay him with Kanji's help, even though she claims that her modelling should cover it all. In the end it turns out he is just using her and his selfish nature is revealed when, in danger, he chooses to save his car over saving Sae. All Sae's ups and downs makes her realize that Momo, Kairi, and Kanji are the only people that actually care for her.