Psychtanium

Psychtanium is a metal alloy that has a crystalline structure, somehow infused with minerals such as quartz. It's knowledge and creation process was a gift from an alien race that was temporarily partnered with humanity. The main property of this alloy is it's ability to absorb and react to a psychic imprint of whoever, is touching it. However, with time, humanity learned to make a variant that would respond to the first touch and permanently stay as what it had turned into.

Upon this discovery, the new found partner of humanity from the stars, became different in their behaviours. They were eventually forced off-planet through a long battle. Now with our Gaia temporarily out of threat, we did what all over humans do when defending themselves do: build giant fuck off weapons to slaughter the enemy, that are overly complicatedly explained.

That's right. Psychtanium's psychic imprint prowess, was used to build mechas. The alloy's materials are somewhat easy to find or make, but the process is costly, meaning it's a fair while between each mech's construction.