She is the youngest of the siblings, and the third Crusnik. A member of the Alliance Aerospace Navy, Seth viewed Lilith as a mother figure and holds Abel in great sisterly affection. When Cain was fatally wounded in an accident, she injected him with 100% Crusnik, restoring his life, but changing him for the worse. After he killed Lilith during the war, Seth joined Abel in confronting him and was the one directly responsible for shooting him out of an airlock.As a child, she seemed to have an aversion to Cain for some reason, taking Abel's side during an unexplained argument of theirs and going so far as to shrink somewhat into Lilith in fear when Cain later appeared.
Source from Wikipeida