Parashat Vayeshev tells of Yosef's brothers' hatred toward Yosef, which resulted in their casting him into a pit when he approached them as they shepherded their herds in Shekhem. The brothers initially decided to kill Yosef directly, but Reuven, the oldest of the brothers, persuaded them to cast him into a pit, instead, with the intention of later rescuing Yosef from the pit. Thereafter, however, the brothers decided to sell Yosef to passing merchants, and thus when Reuven returned to the pit, he discovered that Yosef was not there (37:29).
The Midrash Bereishit Rabba (84), as cited by Rashi, famously raises the question of where Reuven had gone during the time when the brothers lifted Yosef from the pit and sold him to the merchants. The Midrash answers that Reuven had left to pray as part of his ongoing efforts to earn atonement for his sin with Bilha. In this context, the Midrash speaks of the greatness manifest by Reuven through his continued repentance, and tells, "The Almighty said to Reuven: Never before had somebody sinned before Me and repented – you are the first to introduce repentance!"
Many later writers addressed the question of how the Midrash could speak of Reuven as the first man in history to repent. It is commonly assumed that Adam Ha-rishon repented after partaking from the forbidden tree in Gan Eden, and that Kayin similarly performed teshuva for murdering his brother. What aspect of Reuven's repentance rendered him "the first to introduce repentance"?
The Ketav Sofer suggests that Reuven was the first sinner who might have been tempted to absolve himself from the need to repent on the basis of a mitzva he performed. As the brothers plotted Yosef's murder, Reuven intervened and successfully – so it seemed – averted this tragedy, thus sparing his father grief. After performing what appeared at the time as a heroic, life-saving act, Reuven could have considered himself cleared of the incident with Bilha, having made amends by rescuing Yosef. Instead, Reuven continued the process of repentance and repeatedly beseeched the Almighty for forgiveness. It is in this sense, the Ketav Sofer suggests, that Reuven was "the first to introduce repentance." Adam and Kayin still had no other mitzvot to perform, and thus naturally their only recourse to atonement was through repentance. Unlike Reuven, they did not face the temptation to clear their consciences by performing a different mitzva.
Rav Yaakov Neiman, in his Darkhei Musar, suggests a different approach to explain this comment by the Midrash. Chazal explain that Reuven acted as he did with regard to Bilha in order to defend the honor of his mother. After Rachel's death, Yaakov moved his bed into the tent of Bilha, Rachel's maidservant, indicating that he now afforded Bilha primary wife status in Rachel's place. Reuven, Leah's eldest, saw this measure as an affront to his mother, and sought to defend her honor by moving his father's bed to her tent, which constituted a grave demonstration of disrespect to Yaakov. Reuven's wrongdoing differed somewhat from "conventional" sins in that it was not the result of a temporary lapse, or a moment of weakness or lack of restraint. It rather resulted from misjudgment, from the failure to weigh the importance of defending Leah's honor against the respect owed to Yaakov. Most people in a situation such as this would justify their behavior and fail to acknowledge that they acted wrongly. The teshuva that Reuven pioneered was sincere, heartfelt repentance for so-called "innocent" mistakes, sins that could be given a veneer of justification, or even of a meritorious act. His repentance thus sets the precedent of resisting the instinct of self-justification, and acknowledging wrongdoing even in situations where a sinner could find a basis for insisting upon his innocence.