Cliff is the son of Arthur Widmore and twin brother to Alexander. He was born 23 minutes after his brother on August 16th (16/8). He is described as having dark blue eyes, being right handed, and being about 35 years old. He married Shannon Widmore, but not long later she committed suicide. On the same day as her funeral, Alexander disappeared.
Four months later, Cliff hired investigator Paul Artisan to find his brother at whatever cost. He also hired Pru Cunningham to tail his stepmother, Vivian Widmore, after he suspected that she was cheating on his father.
In the book, Cliff is acting CEO of the Widmore Corporation. While Arthur is still in the role officially, he leaves day-to-day operations to his son.