Oriko's ability to foresee the future gained her the understanding that magical girls can become witches, and because of that, a terrible future could await the city. Due to that vision, as well as her upbringing as the daughter of a politician, Oriko feels responsible to prevent that possible future, at all costs. Oriko smiles as she lies to Yuma, to manipulate the young girl into contracting with Kyubey. She convinces her close friend Kirika to do the dirty work of killing other magical girls. Oriko feels those means are justified by her end goal of protecting the peace she has achieved in her life with Kirika, as well as bringing peace to the world. She seems to be highly influenced by her political family in her ability to exercise Machiavellian tactics with much personal charisma, all for the sake of a lofty goal.
Oriko shows a gentler side in her personal nature with her close friend Kirika. She enjoys teasing Kirika, but deep down, Oriko cares greatly for her. She'll even let Kirika sleep in her lap after a battle. Oriko also shows devotion to her family, from when she was a little child helping out her father in his campaigns to now doing what she can to help make her father's dream of a peaceful world come true.
Sadness Prayer explores more of her feelings and motivations from Oriko Magica and shows how she became more cold and ruthless as a result of the death of one of her close friends. In Symmetry Diamond, however, where she does not commit such extreme crimes nor have her close friend die, she is much friendlier and downright heroic in comparison.
When testing out her magic for the first time, Oriko sees the vision of Mitakihara City in ruins, destroyed by the witch Kriemhild Gretchen. Oriko knows that the witch has to be stopped, but it is impossible to destroy her once she's created. She is then struck by another vision of a young girl, Yuma, and proceeds to tell Kyubey about her potentials as magical girl candidate in order to distract him from attempting to make a contract with Madoka.
She appears again when Kyoko has gone to fight a witch, leaving Yuma behind at their hotel room. She speaks to Yuma and suggests that Kyoko may already be dead, playing on Yuma's fear of being alone. Yuma rushes off to find her friend, leading to a chain reaction of events that ends with her becoming contracted. Kyoko swears she's going to make Oriko pay for making Yuma into a magical girl. At the end of the chapter, Oriko asks her friend Kirika over the phone to begin killing magical girls in order to provide further distraction for Kyubey and the other magical girls while she sets her plan into motion.
In a flashback, it's revealed that Oriko is the daughter of a politician, Hisaomi Mikuni. As a young child, she helped him in his campaign. Now she wants to make his dream of bringing peace to the city, and then to the world, come true.
While in her father's rose garden, she and Kirika prepare to have tea. A witch attacks them, and as Kirika fights it, Oriko prepares the tea. Kirika ends up smashing their table and ruining it, so the two just sit together, Kirika resting in Oriko's lap. Oriko muses to herself that if Kirika wasn't there she would have broken down long ago, and then has another vision. This tells her that she's going to have to ask Kirika to fight again, to protect their world. During Kirika's fight with Mami, Oriko has a vision that Kirika will be mortally wounded and intervenes to rescue her. Mami recognizes Oriko from Kyoko's description, to which Oriko responds that they will meet again and Mami will "realize how foolish [she's] been". She leaves without a fight, but Mami is nonetheless stunned as she could sense very powerful magic emanating from Oriko.
Back at Oriko's house, she is attempting to nurse Kirika, who refuses medical attention. Kirika also confesses that, afraid of being rejected by Oriko when they met, she wished to change her personality. Oriko tells her that she can never forgive the deception, but as both she and Kirika are smiling, it seems that they have no regrets. She then discovers that Kirika's Soul Gem has been cracked and begins crying.
As the fifth chapter begins, another flashback of Oriko's past is shown. She was a highly-respected young woman who excelled at almost anything. However, her world was turned inside-out and her reputation sullied when it was revealed that her father had been embezzling money, and he committed suicide to escape prosecution. Alongside Kirika, Oriko hijacks the announcement system at Mitakihara Middle School and gives a speech suggesting that she didn't stop respecting her father in spite of that. She cryptically warns the students about the presence of witches and how magical girls must defeat them for the greater good. The school is then enveloped by a witch's barrier, as familiars attack and devour the people inside. Oriko and Kirika are confronted by Homura at the heart of the barrier world. Oriko reveals that she knows Homura from her vision, and has been trying to change the fate of Mitakihara Town ever since she received it. Her goal is to kill Madoka before she becomes a magical girl and then a witch capable of destroying the world. This was followed by Kirika's witch attacking Homura, who has been joined by Mami, Yuma, and Kyoko in order to protect Oriko from being potentially injured or possibly even killed. Upon learning the truth, the other three are shocked and fight much more poorly, causing Oriko to pity their inability to accept reality. The nature of Oriko's wish is revealed. After her father's suicide, people turned their scorn to her, sending her death threats and barring her from the school presidential elections. Oriko became a social shut-in, convinced everyone had betrayed her (which is not surprising given the way so many were treating her for the "guilt by association" from being her father's daughter), and wondering what purpose she served in life if everything had depended on her father's political standing. At this point, Kyubey had appeared and offered her a contract that would define her existence from then on. Oriko wished to have a reason to live, which resulted in her precognitive abilities. She tells Homura that they are different, because Oriko wants to save many people as opposed to Homura, who is trying to protect only Madoka. She then fights Homura. Kirika is defeated by the other three magical girls working together, driving Oriko into a rage. She only becomes more desperate when Kyubey appears, announcing that he's managed to see through Oriko's plan: using Yuma's contract and the murder of other magical girls to distract him from Madoka's potential. Oriko's Soul Gem is close to exhaustion, but she continues to attack Homura, aiming to make her use up enough magic so that she cannot defend Madoka. When Kyoko throws her spear at Kirika's body, Oriko jumps between them and is impaled, displaying her genuine care for Kirika. With the last of her strength, she throws a shard of Kirika's witch body at Homura, which misses, before finally dying.
However, it turns out that Homura was not her target. Instead, Oriko hit Madoka, who had been coming to help Homura in the fight. Madoka is struck in the heart and killed instantly, causing Homura to create a new timeline. The last shot of the manga is Oriko and Kirika, holding hands in what is implied to be the afterlife, happy they are finally at peace and together still even then, despite all that has occurred.
- Her first name means "to weave" (織), "white jasmine" (莉) and "child" (子). Her last name Mikuni (美国) means "beautiful country" or "country of beauty".
- Despite being the titular character, Oriko is not featured on the first volume cover of Oriko Magica. She does, however, feature on the cover of Extra Story and the cover of Sadness Prayer's first volume.
- Kirika and Oriko are exactly 121 days and 3 hours apart in age. Oriko being the older of the two.
- Oriko was the student council president and top member of her class in Shirome Middle School.
- Oriko also appears in Magia Record.