There's no such thing as too much tragic backstory. Make their family dead, their hometown, no, their whole COUNTRY or even PLANET, GALAXY OR DIMENSION obliterated, so they're the sole survivor. Have them go through a lot of traumatic experiences just for the heck of it. Make them cursed with whatever (maybe everything they touch turns into jello, idk), make them find their new family, then kill them off as well, because no happiness allowed to our wonderful OC. Oh, and the OC's personality? Make them a bubbly and happy person, remembering their sad past and turning stuff into jello only when it's convenient to the plot.