Apparently...
Once upon a time, there was a goddess who was saved by a Shinigami from a Hollow attack.
Eventually, their friendship blossomed into something more.
Soifon, the goddess blessed Yoruichi, the shinigami with the ability to remember all her past lives because Yoruichi doesn't want to forget Soifon.
Since they're both women and can't have children. They've spent most of their time creating and inventing tools/weapons.
One time, Soifon blessed her with shapeshifting, superb speed and strength, making Yoruichi on par with other demigods which angered the god father.
He cursed Soifon to always live a painful life, die, and be reborn without memories.
Yoruichi remembers all their past lives.
Soifon's memories resets everytime she dies.
Yoruichi can always make her remember who she was but Soifon's life will shorten into 2 to 3 years. (which is relatively short if you're a shinigami)
All this time, Yoruichi is the one always waiting for Soifon...