Diane Holmes was a pilot who flew into the Cardiff Rift. She was the lover of a married man prior to going through the Rift.
Diane and Owen Harper had a relationship, but she decided to fly away and try to find the Rift again, to get back to 1953. Whether she succeeded, or if she was killed or otherwise lost in the attempt, remains unknown. (TV: Out of Time)
Owen activated Torchwood Three's rift manipulator with the intent of forcing the Cardiff Rift back open, hoping to find Diane again. Ianto Jones tried to stop him, but failed after a violent scuffle. (TV: Captain Jack Harkness)