Why was Reuven not present when they sold Yosef? Where had he been?
Rashi #1: It was Reuven's turn to serve his father that day.
It says (Makos 23b) that Yehudah's admission about Tamar inspired Reuven to admit! Moshav Zekenim, Hadar Zekenim - Reuven already was repenting. He publicly admitted only after Yehudah did.
In fact, he hoped that, if he returned Yosef to his father, he would forgive him for his sin (Targum Yonasan).