She is Ace's older sister.
They lived together till Garp thought it was best to separate them.
He takes Mikael and leaves Ace, Luffy, and Sabo with Dadan.
What he didn't know was that she was going to meet Shanks on the island where he left her and that he would get a new grandson.
She ended up with Trafalgar D Water Law.
She bandaged him up after he landed on her island.
He was at sea after Corazon, Rosinante, died in order to save him.
Law is older than her, and they became brother and sister too.
She doesn't sail with Law, because she promised Ace she'd sail with him before she left. Mikael is scared of thunderstorms became when Grap took her away from her brothers, he left her in a bad storm, all alone, and she has panic attacks because of it.
She had a run in with Marco the Phoenix.
Her bracelet from Shanks glows, but she still puts up a fight when they meet.
Whitebeard wanted her as his daughter after hearing that she put up a fight against Marco without a devil fruit.