In My Hero Academia, some of the heroes and villains echo each other like counterparts. For example, Izuku Midoriya is All Might's protege, and that makes him a bit like Tomura Shigaraki, the apprentice of All For One (and they both wear red shoes). Ochaco has her own villain doppelganger: Himiko Toga.
For all their similarities, however, Ochaco and Himiko ended up choosing very different paths. While Himiko is self-centered and hedonistic, Ochaco drew inspiration from Izuku to fight for others and make other people happy, starting with her parents. Her life story brushes against Himiko's, but the two girls are hardly identical.
Finding A Personal Hero
Both Ochaco and Himiko seek a personal hero, or even a savior, whom they can latch onto and emulate, as though balancing out inner insecurities or doubts. Ochaco seeks to negate her private insecurities by finding a hero in her own life who can guide her. She isn't looking to All Might for guidance -- she wants Izuku, her beloved classmate. Over time, both consciously and not, Ochaco seeks to become like Izuku, from his courage and selflessness to his sheer grit and vision of becoming the top hero.
In fact, Ochaco told herself "I'll win, and be just like Deku!" as self-motivation while fighting Katsuki Bakugo in the U.A. sports festival. Gradually and surely, this goal is based not just on a need for a personal hero, but genuine love, too.
Himiko takes this emulation a little more literally. Her Quirk allows her to actually assume the likeness of another person. She did this to the hero killer Stain, putting him on a pedestal and seeking to become just like him. Himiko is even willing to kill the original to become like them, and she can become obsessive and fixated on her chosen hero. In this case, she replaced her passion for Stain with her fixation on Izuku, and her burning desire to know everything about him and become him.
She claims that both she and Ochaco are "a maiden in love" who need to have their hero close by, no matter what, and their lives are incomplete otherwise. Himiko is partly right, but she would be wrong to think that Ochaco would totally end up like her.
Why Ochaco & Himiko Fight
These girls are similar in that they need a personal hero and pursue those idols, but the reasons are very different. Ochaco grew up with the potential to become a villain, just like Himiko, but the goodness in her heart steered her in a different direction. Her family struggles, losses to classmates and her fixation on Izuku could have driven her to desperate measures to get ahead and descend into villainy. Her parents need money, and she needs a quick way to become more powerful, but never once was Ochaco tempted to rob a bank or join the League of Villains or the yakuza.
Instead, she followed Izuku's example and aimed to reach the top of the hero world the honest, old-fashioned way. Hard work, smart goal-setting and patience are the routes Ochaco chose, and she is doing all this to support her parents and pay them back for raising her and protecting her. She fights only for others; personal glory and fame never even occurred to her.
That sets Ochaco apart from Himiko, who is clearly fighting for no one but herself. Many of these villains, Himiko included, fight for their own glory, power or wealth, from Stain trying to remake the hero world in his own image to bank robbers and anarchists who delight in burning the world down. Himiko is like that too, getting a thrill out of becoming just like her idols, only to destroy them and revel in a stolen identity. So far, villainy seems to be just a game to Himiko, or perhaps a way for her to run away from herself, whether due to feelings of shame, self-loathing, inferiority or just boredom.
So far, the anime has not made Himiko's motives totally clear, but based on the character's methods and speech, she is on a totally different path than Ochaco Uraraka, the other maiden in love.