17. What is the worst thing you have put your OC through story-wise? For Sera!
OK, soooo her father's death during the Calamity is bad cause he was a pretty laid back person who didn't really strive for anything. And then he never even makes it to Carteneau. He simply dies this meaningless death from a stray meteor taking out his company. So S'era develops this resentment towards him and this drive to achieve something, anything, before she dies. And it eventually mutates into a pretty basic fear of death.
Then the end of 2.x is pretty bad because S'era ends up killing a good friend of hers who had joined the Crystal Braves. The grief and anger from that lead straight into her becoming a Dark Knight.
Shadowbringers is also pretty bad cause to start, her best friend / adopted sister Marie is one of the Scions that falls unconscious. Then she gets warped to the First, learns the WoL is dead and she's now the Hero. And so on top of all the crap that happens in ShB, you've got a person who's struggling with the idea that she's a replacement hero/friend and is she really living up to that and could the actual WoL have done better, and oh hey! fear of death coming into play with that whole "turning into a monster" thing.
BUT, I think the worst might actually be the thing I haven't fully worked out or even really decided on? Which is that during the Werlyt storyline, S'era is captured and forced to pilot the Diamond Weapon, which is still using the Zenos core auracite.
So you picture Overmind activating, with that jackass' personality behind it, and then S'era's Echo ends up interacting badly with it? Again, I haven't gone too far on this one, but I think you briefly end up with the closest thing to Ascian Azem here.
I don't know if that situation happens to Primary storyline S'era (WoD S'era), but I think it definitely does to WoL S'era, and is a good reason why her state of mind in Endwalker is pretty bad. (Oh hey look, context for storylines!)