It is Juror Nine, the elderly man on the jury, who changes his vote from guilty to not guilty at the end of Act One, and he is the first juror to do so. Juror Nine changes his vote to support Juror Eight, who wants to discuss the evidence before reaching a verdict. Juror Nine says he wants to hear more, also. By the end of the play, Juror Nine has played a significant role in sifting through the facts of the case to reach a fair verdict.
At the end of Act I, Juror Nine changes his vote to not guilty. There is a secret ballot at the end of Act I, and the person who reversed his vote is not revealed until Act II. When the other jurors press him about why he changed his vote, Juror Nine says of Juror Eight, "It takes a great deal of courage to stand alone even if you believe in something very strongly." Juror Nine believes that Juror Eight showed the courage of his convictions, so he thinks the other jurors should listen to him. Juror Nine later reminds the jury that the accused must be guilty beyond a reasonable doubt.
Juror Nine is an older man, and he is meticulous and observant. He later recalls that the old man who allegedly witnessed the crime was wearing a torn jacket and had two canes. Later in Act II, he says of this man, "A man like this needs to be recognized" (pages vary by edition). Juror Nine is a careful observer, and he understands the motivations of this witness better than the other jurors do.