@yoroshikue
Explanation chronologically.
The boy and the girl summoned together because the boy try to save the girl from suicide even trying to shield her from train when it seems futile in the previous life. Because of this in the new world they body swap. Boy become Witch and Girl became Swordsman. Though the they said they cannot remember their previous life girl/swordsman remember at the end that boy/witch try to save her and girl/swordsman promise to protect boy/witch now.