Aidan Keller is the 10-year old son of Rachel Keller and Noah Clay. Apparently he and Katie were rather close and used to play together. Katie's death was a great impact on Aidan and he confessed to his mother that Katie knew about her death. After Rachel saw him watching the videotape, she became desperate to unravel the mystery behind the tape.
In The Ring Two he was possessed by Samara and started to develop hypothermia. When he and Rachel went to an amusement park, he slipped into the bathroom and took several pictures of himself. That's when Samara possessed him.