Author's Note

A bit of explanation:

No, Deku can't control his personality changes. It's hard to explain, though. Do you know the voice you think with? Mine sometimes tells me an unpleasant fact I do know, but tried to obliterate. Deku is the 'unpleasant fact' voice when Izuku is the main personality and contrariwise.

Because Izuku has such mental issues, his 'unpleasant fact' voice can speak aloud. This psychological stress lets him lose physical control of himself a bit, and his eyes are left uncontrolled. So they flop around.

As you may know, second personalities are a way of a mind to protect itself with a 'meaner' one. These personalities emerge in moments of psychological stress, pain or through sudden mental urges. Because Deku wanted to protect them and be in control before, but Izuku longed to talk to Bakugou so much, the change could happen quickly. Deku just stopped holding on.

The 'holding on' process was harming himself. The Izuku part has a lower pain tolerance than the Deku part, and when he feels immense pain, the urge to let Deku up front emerges again. In the end, Izuku wanted to be with Bakugou so badly, Deku had to make him not want to be there all the time.

Deku could only take the main control because they had to endure so much emotional and physical pain, they wanted to protect themselves all the time. At first, they contained that around Bakugou aswell, because he was an asshole earlier. However, the Izuku part realized Bakugou had gotten nicer.

Deku kept being a Yandere.

I hope you understand my way of portraying Midoriyas personality problem, and if you have DID and I have offended you, please forgive me! I didn't mean to.