bethany was a patient in radley, roommates with lesli stone around the time charlotte was there as charles.

mrs d. was having an affair with bethany's father and bethany knew this so mrs d. used to blackmail her with trips out of radley e.g. to horse stables and make her call her 'aunt jessie' to prevent suspicion.

bethany knew that charlotte had out privileges (since her mum was on the radley board) and so the night alison disappeared bethany stole charlotte's clothes (an exact replica of alison's as mrs d bought charlotte what she bought alison) and bethany escaped posing as charlotte.

no one knows how she ended up at the dilaurentis house that night (possibly to confront jessica about her affair with bethany's father) and then mona mistook her for alison due to the clothes, hit her on the head w a shovel, panicked and ran and then melissa assumed it was also alison and that spencer had hit her so she buried her to protect spencer

there has to be more to her story and why we are never shown her adult face etc