seize the day
When Sif was very young, she and Thor ended up playing together a lot. She didn’t think much of it, except that Thor could be pig-headed and bossy, and honestly she found herself teaming up with his little brother against him more often than not. Usually for spite, because he always wanted to play hero, while Loki would play villain and Sif would play a damsel to be saved. The prize. She hated the roles he gave her, so, when Loki came to “kidnap” her, she tended to work with him until Thor ended up on his back in the mud, villain and damsel standing over him victorious.
Sif knew she and Thor were betrothed. She knew it, but she found the idea unpleasant. As far as her young mind was concerned, she should be marrying Loki instead. They worked together better, and he was more fun than his brother.
Eventually, things changed, as they must. Whether he was sick of losing, or whether he learned some truth of leadership, Thor realized that if he offered to make Sif his knight instead of his damsel, she would fight for him. Loki’s days of winning their childhood games were over.
The morning after she first sided with Thor, she woke up and screamed and screamed. All her long, blonde hair was gone, cut off. The fuzz that remained was black as the night sky, black as Loki’s hair. Sif shook for several hours afterwards. It wasn't the missing hair but the feeling of helplessness she hated so much. She'd gone asleep and woken up changed. Assaulted. It was months before Sif could go to bed alone again, and she would forever be a light sleeper.
No one could prove it was done by Loki, but everybody knew. Sif challenged him to einvigi, but they were children so Ullr refused to let them fight. Instead, she waited until there were no adults around and punched him as hard as she could.
She broke his nose. But she also lost the fight that followed. Loki had been receiving formal training since he was very, very young. Sif was a girl and so had not. The next day, wrapping a broken wrist she dare not bring to the healers or be revealed as the one who broke a prince’s nose, she started training herself.
She was never going to lose another fight ever again. She was never going to be so vulnerable to cruelty ever again.
Her quest to be a warrior was scoffed at by almost all of Asgard, but support came from an unlikely place, the princes. Thor thundered his support publicly and often, but Loki helped too, helping her to find weapons and teachers. In time, she forgave the younger prince. But she could never bring herself to trust him.