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Mild Curious Village spoilers under the cut
I'd like to add that someone harming Luke or Flora would push him over the edge. I'm sure he would for Katrielle or Alfendi when they were younger, or Emmy if she was in that situation, but really I've only seen them when they are old enough to defend themselves, so I can't say. And it wouldn't be in a controlled way, which I think he would do in defense of himself or others, especially innocents. It would be in a you-have-hurt-the-child-in-my-care way. Flora is alone in terms of human relatives, and Luke is usually FAR from home, so I wouldn't be surprised if he had a papa bear instinct specifically for his charges.
There's a fanfic that takes place during the Ace Attorney crossover game where Layton rages. Spoilers for the game, but if you don't care, I didn't have too hard of a time following: https://archiveofourown.org/works/49064023
It is finished and very well written, but heed the tags and take care of yourself as always.
Sometimes I like to think about what it would take to make Professor Layton ever hit someone. Like how far does he need to be pushed? Or would he never at all?