"When the gatekeeper to hell asks you who sent you, tell him it was Shinobu Jacobs."
No More Heroes
Shinobu is first encountered at Santa Destroy High where Travis Touchdown finds her in class, she is initially surprised to be challenged to a Ranking Battle in the middle of class. She tells Touchdown to wait outside while she kills a small group of students who overheard Touchdown challenging her to a Ranking Battle. Shinobu is impatient and seems to want to get the fight over with. However when Travis pulls out his beam katana she becomes angry accusing him of killing her father. It is likely that Touchdown's twin brother Henry was the one who killed Shinobu's father, as it isn't until Touchdown activates his beam katana that Shinobu's overall tolerance of him harshens, proclaiming she will finally avenge her father, as Henry wields a beam katana similar to Travis'. The two square off, during which Touchdown severs Shinobu's right hand with his beam katana. Touchdown then reveals he had idolized Shinobu's father so much so that he watched all of Jacobs' tapes until they wore out, but had never actually met him in person. Shinobu insists he's lying and tells him to kill her, Tocuhdown ignores Shinobu's cries, opting to spare her life so the two can duel again in the future.
Although it is implied that Touchdown could not kill her because of his discomfort in killing a woman, and reinforced with his victory against Holly Summers, in the pre-fight phone call Sylvia sardonically defines Travis as nothing but a cold-hearted assassin "...because that is all you ever do. What? Am I wrong?". Travis begrudgingly agrees but his actions make him think twice when he realises Shinobu is not a killer for superficial reasons but for reasons similar to his own: they both want to find who killed their parents. Shinobu later appears with her right hand reattached, to save Touchdown from being killed by Jeane in the climax of his final Ranking Battle after it is assumed Shinobu had followed Travis in secret to learn of his reasons up the ranks. This 'kindred spirit' relationship would play a profound affect in shaping her for the sequel.
In the three years between No More Heroes and Desperate Struggle, Shinobu has become the most prestigious assassin in Asia. She reunites with Travis after he defeats Dr Letz Shake, showing him her belt she won from her fights in Asia and revealing that Sylvia told her to come back to Santa Destroy and help Travis out. Travis claims it's his war and that he doesn't need help, but Shinobu says it's too late for any changes to be made as she has already killed two other assassins, the first being Million Gunman. Before her ranked fight with new Destroyman, she visits Bishop's grave, and meets his unnamed friend. He gives her a mysterious gift for Travis, which is later revealed to be the rose nasty. Shinobu then proceeds to her next ranked fight with New Destroyman. After her fights, she finishes explaining how she defeated the two assassins and then attempts to seduce and kiss Travis. Travis claims he "[feels] like that pervy teacher in a porn", showing his discomfort for taking advantage of a subordinate. He tries to get her to stay but afterwards, but she sadly leaves.