Previous Story:
Kasumi is Ayane's half-sister (same mother, different father) and the 18th generation successor of the Mugen Tenshin Tenjinmon clan. In an attempt to look for her brother, Hayate, she became a runaway Shinobi. Before that, just after the first "DOA World Tournament", she was captured by the Superhuman Development Organisation. While held captive, she was being cloned and "Kasumi alpha" was created.

DOA3 Story:
Though Kasumi finally met her long-lost brother Hayate in the last tournament, there was no way a runaway Shinobi could return home in peace with her brother. Day after day she is forced to defend herself from attempts by highly-skilled assassins. Then one day Kasumi learns that Hayate is to be at the "DOA3 World Tournament". She decides to show up at the tournament so that she can see her brother just one more time.