this is a spoiler from a user below in the comments @sweet_shortbread it basically explain...

this is a spoiler from a user below in the comments @sweet_shortbread
it basically explains why he treated her like that during their time in school
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SPOILER
There are some clues already in the first book that there is something strange about Lockmann. When they meet for the first time as kids, Nanalie thinks it's a bit disturbing how Lockmann was able to quickly switch gears between smiling brightly and being expressionless.

When Lockmann was little, his magic power was out of control. Lockmann's magic nearly killed his mother at his birth, and continued to be so strong and uncontrolled that the adults around him couldn't keep it contained. Any expression of emotion from Lockmann, whether happy or sad, just caused more outbursts of magic. So Lockmann understood that his emotions were a problem and stopped speaking unless he had to. Dr. Aristo persuaded Lockmann's parents to let him take care of Lockmann. Lockmann's early years were spent isolated in Dr. Aristo's facility, surrounded by power-draining tools. That's how he and Aristo know each other.

This whole experience was very psychologically damaging to him. So that's part of who the real Lockmann is. The self-control is something he's been forced to learn out of necessity. It's mostly just Nanalie who causes him to show strong emotions.

Nanalie seems to be more of a late bloomer, but the fact that her hair and eyes turned blue was a clear sign that her body also carried more magic than such a small vessel could really handle. At that time, Lockmann first thinks to himself he should just leave the matter alone; magic is a struggle, that's just how it is. But then he imagines Nanalie's lonely back being made to leave the classroom and her becoming isolated in a room somewhere. He can't stand the thought of her becoming like him. He thought that he couldn't forgive himself if the smile disappeared from that girl.

That's why he was always fighting with her. Lockmann's fights with Nanalie were meant to provide an emotional/magical outlet for Nanalie to blow off steam through. She spent so much energy fighting him, there was no excess left to overflow. And she never did have any issues controlling her magic. The first and only time she experienced a pestrokraive episode was when she saw Lockmann badly injured at the end of volume 2.

To sum up: Lockmann actually spent all those years trying to save Nanalie from becoming as damaged as himself.

link to the thread:
https://forum.novelupdates.com/threads/i-want-to-be-a-receptionist-of-the-magic-world.107858/page-3