Rose is one of five friends who landed on the island with Daphne. Like Daphne and the others, she was taken prisoner and forced to undergo the Trials. She did not make it out of the starting Trial area, however, and her corpse can be found near the Watchtower. Her death is presumed to be caused by a reaper Executioner, as she is cut cleanly in half.
No executioners wander the area near where Rose's corpse is found. Her body was either placed there after her death or the responsible executioner moved to patrol a different area, perhaps searching for other aspirants. A reaper executioner can be located in the Temple Ruins, which is not far from the Watchtower.