Dawn Samos is a pure-blood first introduced in Half-Blood but not seen until Pure at the party in the New York Covenant. She is Lea's half sister and legal guardian after her parents (Lea's father and stepmother) were killed by Rachelle Andros while she was a daimon.
Alex thinks she likes Aiden which makes her dislike Dawn but saves her life in the daimon attack in Pure anyway. After the fight, Aiden uses a compulsion on Dawn and another pure to make them forget about Alex killing Telly's pure-blood guard in self defense.
Not much else is known about Dawn except that she took her parents' seat on the Council and was an advocate for half-blood rights. She was killed, along with the other council members, by Seth, and by extension Lucian, when they tried to take over the council in Deity.