Russ Brennan born Kyle Keenan is Temperance Brennan's brother, who left her when she was fifteen years old (when he was nineteen), shortly after the disappearance of their parents.
At seven years old, his father told him he was never to use the name Kyle Keenan again (and that if he did, it would get his sister killed) and his new name was Russ Brennan. When they were children, he had a close relationship with his baby sister, Temperance, who idolized him. After he left home, Russ committed various misdemeanors and felonies, and worked as a mechanic in North Carolina.
When a Jane-Doe body was identified as Russ and Temperance's mother Ruth Keenan in "The Woman in Limbo", Booth tracked Russ down. Russ gave Angela information that allowed the team to arrest Ruth's killer. Bones' relationship with Russ after he left was initially unwilling, due to her feelings of abandonment. At the end of the episode, Russ and Temperance made up, and the latter enjoyed referring to him as "my (her) brother" in the next episode.
Russ seemed to live a peaceful life for a while after this. However, in "Judas on a Pole", Russ's and Tempe's father resurfaced, due to the fact that a rogue FBI agent was trying kill Russ. Though Max himself killed the sniper, Russ helped his father escape from the law and violated the terms of his parole multiple times on/after the case, which caused him to go into hiding.
Russ did not resurface until "The Knight on the Grid" after learning his stepdaughter Hayley had been hospitalized with complications related to her cystic fibrosis. He was arrested by Booth and subsequently sentenced to thirty days in the county jail, with the term of his probation extended and amended to include the wearing of an ankle monitor. In the next episode, "The Santa in the Slush," Bones manages to get a trailer for Max, Russ, Amy (his girlfriend/fiancee), Emma, Hayley, and herself to spend Christmas in.
It is unknown exactly when Russ was released from prison; however, it was sometime before the events of "The Verdict in the Story," the episode in which Russ's father Max was tried for murdering Robert Kirby, the Deputy Director of the FBI. Russ testified on the stand that he did not witness Kirby's murder; however, it is heavily implied throughout the episode that Russ, like his sister, Tempe, correctly believed that Max did commit the murder. In the end, Max is acquitted of the crime.
Russ has not made any further appearances since then; however, he has been mentioned several times. In "The Goop on the Girl," Max stated that Russ was spending Christmas with the girls down in Florida, with the in-laws, implying that Russ may have married or at least become engaged to Amy at this point.