Genii citizen, sister of Ladon Radim. Dahlia was likely a nuclear scientist, as she worked in close-enough proximity to Genii nuclear devices to be one of a very small group of people to become terminally ill with radiation exposure.
Knowing she would not recover, Dahlia offered her life to the Genii cause and traveled with a number of other terminally ill Genii to Atlantis, knowing Dr. Elizabeth Weir would hold them hostage when Ladon did not deliver a Zero Point Module. Meanwhile Chief Cowen had no obligation to return several captured Atlantis expedition members, as every Genii who traveled to Atlantis did not expect to return -- less live much longer.
But Dahlia was wrong when she learned Dr. Carson Beckett had perfected a treatment that would heal most of the people's radiation sickness. When Ladon learned of this he let the captured Atlantis personnel in on his plans to take over the Genii government. In exchange for their lives he would get his people (including Dahlia) and Beckett's medical techniques to whoever needed them.
Coup D'etat - Dahlia Radim offers her life to a Genii cause, knowing she will help to ensure the survival of her people.