[headcanons, yo. got any specific ones? uvu]
Oh my which headcanons which headcanons...
Welll, during his journey with his sister and their group, I can see Mithos often pulling pranks on humans who acted really haughty. Nothing harmful or overly ridiculous. Just little stupid things that he could get away with without getting caught. It was his harmless way of getting back at the people who treated him and his sister like they were beneath them, and while Martel didn't really condone the pranks, she often got a few laughs out of some of them.
Because of the event I'll give another one about before his sister died too hehehe
Mithos used to dislike magi-technology quite a bit. The advancement of magi-technology is what allowed Tethe'ala and Sylvarant to start the Great War, and it is what killed the Great Tree, thus basically killing the planet. So he hated it and wouldn't use it unless he had to. In the beginning he probably even blamed the war more on the technology than the people. Though this slightly changed after his sister died, and eventually he started readily using magi-technology himself to try and reach his goals.
It's like 8am I really hope those make sense!!