William Reibert Mapother, Jr. (born April 17, 1965) is an American actor and former teacher, perhaps best known for his role as Ethan Rom on the television series Lost.

Cousin of Tom Cruise and Lee Anne De Vette.

Graduated from Notre Dame with a B.A. in English.

Was a school teacher in East Los Angeles for 3 years before becoming an actor.

Went to St. Xavier High School in Louisville Kentucky.

His father, William Reibert, Sr., was an attorney and bankruptcy consultant and was a judge in Louisville, Kentucky (1967 - 1970). His mother's name is Louisa.

Brother of Amy Mapother and Katherine Mapother.

Last name is pronounced "MAY-pah-ther"

Is a graduate of the Joanne Baron / D.W. Brown Acting School in Santa Monica, California.

Elected to National Board of the Screen Actors Guild for a three-year term (September, 2007).