Brian "Kato" Kaelin, a bleach-blonde southern Californian with surfer mannerisms, Kaelin offered a sense of humor to the trial. Called "Kato" since childhood, from actor Bruce Lee's character the in "Green Hornet," Kaelin shared the same name as Nicole's dog, which Nicole and O.J.'s children affectionately named after their mother's friend.

An aspiring actor and professional freeloader, Kaelin lived in the guesthouse at the Rockingham mansion, where he often tended the Simpson children. Kaelin was a casual friend to O.J., who talked with him on occasion and allowed Kaelin to continue living at the guesthouse after the Simpson's divorce.

Kaelin's testimony regarding O.J.'s statements prior to the murder, as well as Simpson's whereabouts on the night of the murder made Kaelin an important witness for the prosecution. However, due to his "flakiness" on the stand, Prosecutor Marsha Clark treated Kaelin as a hostile witness, asking Kaelin on the stand if he was "changing his testimony." After his four days of testimony, Kaelin exclaimed, "I'm so relieved."

After the trial, Kaelin began a short-lived career in radio talk-show hosting. Kaelin sued a tabloid for defamation of character and was himself sued by his biographer. Kaelin also appeared bare chested in an edition of the "New Yorker" and on the cover of "Playgirl."