I'll try answering this one for my characters (see
my comment here for a personal answer). This was more difficult than I expected because I don't think of pain alone as a character building event. (I tend to mess with my poor characters' heads more than smacking them about.) So...
Fractures (who has a given name but I can't find my notes)- being caught and buried in a mine quake
Genie- natural childbirth (she wasn't keen on the idea, but her spouse nagged her into it and, as you might expect under those circumstances, it didn't go well)
Cypress (Eliza's husband)- any one of many beatings as a kid
Micky- she grew up with a degenerative nervous system disorder, so pain of any kind was welcomed because it meant she still had working nerves (later her superpower made pain at most a temporary condition, so she doesn't think about it the way a normal person would)
Pete- as an adult, being beaten and nearly killed by his brother at their niece's wedding (Pete mis-gauged how far he could needle his brother; the goal was to be punched, not killed)
Eliza- she'd say it was fracturing her finger playing min-g ball as a kid, but in reality it was when her ship was hit in one of the last battles of the war (Pete wasn't supposed to be able to heal, so he made sure she didn't remember getting hurt)