Every Day by David Levithan
A is a teenager living a most unusual life. A grows and learns and ages, but has no actual body of their own. Instead, they spend each day in a new host body. A does their best not to disturb the lives of those they “borrow” for the day, and generally A’s presence goes unnoticed, with memories of the borrowed day somewhat blurry. A always inhabits someone their own age and in the same geographical area as the day before, but otherwise circumstances vary wildly. A has been different genders, races, social classes, and states of health.