Elected to the New Republic Senate in 11 ABY and had emerged as a Senate leader by 16 ABY. During the Defense Council's discussion of Operation Hammerblow, Marook was the first Senator to openly question the decision to place General Etahn A'baht as the Fifth Battle Group's commanding officer. Based on the Hrasskis concept of promotion through seniority, he saw A'baht, a relative newcomer to the New Republic Defense Force, as an inferior choice to a commander with greater Rebel Alliance ties. Marook also viewed A'baht's experience with the Dornean Navy as limited due to its relatively small size.
Following the diversionary negotiations between Nil Spaar and Leia Organa Solo, Marook was one of three Senators who supported Spaar's position, although he did not flee Coruscant after the Viceroy.
His name is first misspelled as "Cion Marook" in Before the Storm.