<P> The first season featured a cast of five main actors who received star billing . Tom Selleck portrays Francis "Frank" Reagan, the patriarch of the Reagan family, a former Marine, and current Police Commissioner who is notably silent about his political beliefs . Donnie Wahlberg acted as Frank's eldest son Daniel "Danny" Reagan, a hard - nosed NYPD Detective and former Marine who often roughs up his suspects, despite being prohibited to do so . Bridget Moynahan portrays Frank's daughter Erin Reagan - Boyle, an assistant district attorney who abides by the law in her dealings with criminal justice . Will Estes portrayed Frank's youngest son Jamison "Jamie" Reagan, a rookie police officer and graduate from Harvard Law before entering the police academy . Len Cariou played Frank's father Henry Reagan, a retired police commissioner and former Marine who, like Danny, is very vocal about his political beliefs . </P> <P> In addition to the regular cast, the season also featured actors who received the "also starring" billing, appearing in almost every episode of the season . Jennifer Esposito portrayed Jackie Curatola, an NYPD detective at the precinct and Danny's partner . Nicholas Turturro portrayed Sergeant Anthony Renzulli, Jamie's partner and mentor, who had been a partner of Joe Reagan . </P> <P> Several recurring characters appeared in the first season . Amy Carlson appeared as Linda Reagan, Danny's wife and mother of two children Jack and Sean . Sami Gayle portrayed Erin's teenage daughter Nicole "Nikki" Reagan - Boyle (Marlene Lawston played this role in the pilot episode). Abigail Hawk portrayed Abigail Baker, Frank's primary aide . Brothers Andrew and Tony Terraciano portray Sean and Jack Reagan, Danny and Linda's sons . Gregory Jbara acted as Garrett Moore, the NYPD's Deputy Commissioner of Public Information . Robert Clohessy played Sgt . Sidney Gormley, the immediate supervisor of Danny and the other detectives at the precinct . Actress Dylan Moore appeared in the first six episodes as Jamie's brief fiance Sydney Davenport . Bruce Altman portrayed Mayor Frank Russo, Frank's superior . Andrea Roth appeared in four episodes as Kelly Davidson, a television news reporter who dated Frank . Yvonna Kopacz Wright portrayed Ava Hotchkiss, a detective at the precinct and Danny's temporary partner . Noelle Beck appeared in three episodes as Frank's Deputy Press Secretary Sue Connors . Nick Sandow recurred in four episodes as Lieutenant Alex Bello, an officer at Internal Affairs . Michael T. Weiss portrayed Sonny Malevsky, a detective who had worked with Joe Reagan before his murder . Colleen Clinton appeared as Agent Anderson, an FBI agent who attempts to recruit Jamie for her investigation into the Blue Templar . Bobby Cannavale appeared in three episodes as Charles Rosselini, Erin's boss who becomes romantically involved with her . Robert John Burke portrayed Jyle Hogan in three episodes . </P> <Ul> <Li> Tom Selleck as Police Commissioner Francis "Frank" Reagan </Li> <Li> Donnie Wahlberg as Detective Daniel "Danny" Reagan </Li> <Li> Bridget Moynahan as ADA Erin Reagan - Boyle </Li> <Li> Will Estes as Officer Jamison "Jamie" Reagan </Li> <Li> Len Cariou as Henry Reagan </Li> </Ul>

Who plays nikki in season 1 of blue bloods