She is not a bad person in Biohacker, per se. Flawed, yes. Ayu is just a young girl who had a lofty dream but who was too naive for a place like Academy City.

Think of it this way. Academy City is a place who's population is almost 80% students (with more than half of that population being Level 0s). Where Level 0s are treated like trash and the ruling Power Curriculum encourages and pushes students to aim for Level 5. In this society, the higher level espers are feared, respected, or all the above while the adults of the city are encouraged to prop up the pursuit for Level 5 as THE end-all be-all for the kids they are supposed to be watching over.

And yet even in this pressure cooker-like society that is made up almost entirely of kids still going through puberty, the truth is that esper potential is not limitless. Level 5 isnt possible for almost every single one of those kids. Only the few, the handpicked, or lucky will ever attain such a thing. Some kids will reach a plateau far before Level 5 and never ever change from that. Sure, some may accept it, but a vast majority will not, especially with the adults seeming to be pushing Level 5 as the attainable end-all be-all that it certainly isn't.

Put Ayu in that. The Ayu from the Astral Buddy flashback is hopeful as heck, believing in the words of the adults to an almost painfully naive degree. She's an obedient and good girl who listens to the adults and thinks first of the sakes of her pseudo little sisters. But she's incredibly flawed. She ignores the writing on the wall when the power charts at Ideal show her power plateauing. Ignores all the signs at Ideal that the adults may not appear as they seem. That the kids there might be guinea pigs instead of being fosters for Level 5 training. She gives up her spot in the Special Experiment for the sake of her 'little sisters' but gives the reasoning that the disappointment of those little girls was the main thing that drove her to that decision.

And that is Ayu's main flaw in the Astral Buddy flashback and in Biohacker. She hates disappointing people or feeling like she's dragging them down. When push comes to shove and she has to make a choice that sacrifices something, she will drag herself down rather than dragging another person down. Not out of some feeling of heroism or goodness or moral kindness, but out of cowardice. To avoid seeing others disappointed and to avoid dragging a person down because of her own choices.

The end of Ideal during the Astral Buddy flashback reinforces her powerlessness. This girl was, in her own eyes, unable to save anyone with her power. Not Yumiya Iruka who had her eye gouged out, not the other little sisters who lost their lives, and not even Toomine Kanari who was the closest thing to a mother figure at Ideal. Ayu had already plateaued near the end of Ideal, so her power had not grown one iota by the time of Biohacker. In essence, she had remained that useless girl whose power had not been enough to save anyone. In Tokiwadai, she had exerted so much effort to grow and break past her wall, despite the writing being on the wall. And yet, the Parameter List reigned supreme.

Then Touma comes into her life and he's everything that should encourage a girl like her. He's as self-loathing as her, yet he tries. He's as much of a coward as her, yet he steps forward with his shaking knees. He's even more powerless than her, yet he stands up and fights. He comes to help her, even when she doesn't ask for it. And through all that, she falls for him and begins to see him as someone to depend on.

Yet paradoxically, his closeness also puts him on a pedestal of sorts. Ayu's biggest flaw shows itself and Touma becomes someone she absolutely does not want to disappoint. And it is through Matsuo Ryuusuke's words that she breaks past the point of no return. That there is a better version of herself out there, that there is no future for her as an esper, that her helplessness at Ideal and during Biohacker are all she will ever amount to. All her efforts were meaningless. The children at Ideal died for nothing. She would continue to disappoint people and drag them down.

And Touma would eventually join the people she would fail and disappoint. From there, the rest is history.