If she had not done what she'd done, Rukia reasons, many things would have happened, all terrible: the Hollow would have kept going on in its rampage, killing innocents left and right; Soul Society would have been in a full-on panic, lost their cool in a moment of weakness; she would have died and that would maker Byakuya very sad; they would have never met, Rukia and Ichigo, would have never become partners, friends, lovers.
But she did. She gave her powers to Ichigo, knowingly committing a crime against the highest echelons of Seireitei itself, who would eventually send her own brother to capture her and have her receive punishment in Soul Society.
That came later, six months later. Between then and the past was a period of near-normality, sending Hollows down below while sending pure spirits to a better place, Ichigo at her side, her apprentice in her once private practice. Rukia usually worked solo, occasionally with Byakuya or Renji. These days, she went around her business with a cast of characters: from Ichigo to spirit-wielding Orihime; the foreign fighter Sado to the Quincy heir, Ishida.
She shared a bedroom with Ichigo, taught him everything she knew that an intermediate shinigami like himself should know. They shared a drunken kiss on New Years Eve; Rukia found herself unable to look the redhead in the face for a week until Orihime practically forced the two to talk it out.
"Let's try it out," Ichigo suggested, as if their relationship was little more than a new toy, patent pending and waiting for a seal of approval.
Spirits came and went, weeks flew by without mark. Rukia watched Ichigo put a young female spirit to rest, barely six years old and frightened of what lay before her beyond life's limitations. Ichigo held her hand and walked her to her favorite spot in Karakura, the local park; he pushed her on the swing until his arms got tired, and then some. He takes her around town, say goodbye to her grandmother, her school friends, her parents' graves. When she fades away into the morning light, she's smiling like all her troubles have been lifted off her shoulders.
Rukia watches spirits ascend into the beyond like this with Ichigo's guidance on a nightly basis, and it breaks her heart. She would lead him into Seireitei and reconnect him with his dead mother's spirit if she could, if it would take away the pain that flashed across his face every time he led away someone whose parents or loved ones had left before their time (but no one truly leaves before their time, that is what Soul Society teaches her, a lesson she will never teach Ichigo).
She realizes that, in a way, she is playing God: withholding information from Ichigo, playing with his emotions. She loves him in a way she feels she'll never fully comprehend, and knows one day this all must end, either by his hand or her own. She feels the spark of shadow within him, akin to the birthing of a Hollow, but keeps quiet. His heart is not her business, even when it is so utterly devoted to her self.
She shouldn't be surprised when Byakuya comes for her. Rukia only wishes she'd had more time, but for what she doesn't know. She's done playing the invisible hand of fate; she resigns to her own society's form of justice in the end and hopes that Ichigo can forgive her just this once.
The morning before Rukia is taken away, a rainbow ripples across the sky and she feels as if something is coming to take her life, but she shrugs off the feeling and spends her last day in the living world, completely unawares of what awaits right around the corner.