My personal head canon is that each main character is a reincarnation of the last one. I mean why not? I am the champion of all those daedric princes and the gods right?
Kerrianne lay on the small cot in the border hospital, dying of a festering shoulder wound. Her once thick red hair, matted to her forehead with fever sweat. The healers hadn't told her she was dying, but she knew. She had started hallucinating two days ago. Like the monk, slowly making his way up the aisle between the cots. He was dressed a little better than the others, stood a little taller. His face was hidden by the deep cowl of his robe and she couldn't see his hands for his sleeves. Kerrianne was sure he wasn't real as no one else in the tent reacted to his presence. He paused beside her bed before speaking.
"Hello Kerrianne." he said softly as he sat beside her. Kerrianne gasped, nobody had called by name in twelve years. Not since Martin had turned himself into a dragon and sacrificed everything to save the world. Not since Kerrianne had lost her position as the Fighters Guild Master when it was discovered she was also the Listener for the Dark Brotherhood.
"Martin?" she tentatively asked. The monk slid his cowl back far enough to reveal his face to her, the face of a man she had been proud to call friend. He gently took her hand.
"Yes, I am Martin, but I am also Akatosh. "
Kerrianne softly began crying as she realized what was happening.
"I have come to take you with me, for aiding me in defeating Mehrune."
"Will it hurt?"
"No, not really."
"I'm ready to go then."
Martin/Akatosh stroked his thumb over the back of Kerrianne's hand and her eyes closed as her heart stopped. Akatosh gently lifted her weary soul from her body. Cradling it to his chest like a beloved child, He faded from sight.