Hiya! (as a side note alot of this is halo lore due to mythic being a halo TTRPG)
so for my mythic character i started with him being born on a planet called harvest to a widowed mother after my father was killed during the insurrectionist wars, growing up without a father was hard but it also gave him the inspiration to join the United Nations Space Command Marine Corps, getting him shipped off to UNSC Training Facility Bravo situated in the Eastern Provinces of Africa, during his time spent there something rumbled back at his quaint home on harvest, in the year 2502 the first recorded conflict between the humans and covenant forces, attacking during an ongoing conflict with a Insurrection leading the insurrectionists to believe it was some form of UNSC trick or scandal, everyone came to know the reality of the situation as the covenant used a super heated beam to turn half the planet to glass. Fortifications, Bases, Homes and even family turned to dust in the blink of an eye, its unknown why they left the other half Untouched (at the time frame the TTRPG is set its unknown, that information becomes revealed during the events of Halo Wars) Knowing the fear his widowed mother and childhood friends must have felt during that moment drove him deeper into his training with one resolve, make every single alien he saw feel the same fear they did.
and thats the backstory i had for him! using topics of revenge and helplessness to give a character a reason why when every other man around him buckles in fear at the sight of a 7 foot tall alien he can run into a fight feeling only hate