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Marco Lesa is a Returner from the Medivac 12 who hides away on the Phaeton 5. According to his daughter, Jenny, he died due to the Cylons (Battlestar Galactica 0), presumably prior to the Fall of the Twelve Colonies.
After evacuating the Medivac 12 in zip pods (Battlestar Galactica 1), Lesa lands on Phaeton 5 and reunites with Jenny, who apparently resided on that ship. He protects her from serious injury after he is critically injured in an Earth Protectorate bombing.
Commander Adama asks him to help in a "cause" to find Earth. In reality, Lesa's "help" involves being tagged as with each Returner with a small explosive device in the event that they behave as humanoid Cylons (Battlestar Galactica 6).