We (a family with 2 kids of 15 and 17) had a great 6 days at Despina's place. Despina picked us up at the airport and brought us to Kamari centre for a drink/ lunch (because we were early and the apartment was not yet ready). Afterwards she first drove us to the apartment and she offered us to bring us to a large supermarket. That was very nice of her of course!
The apartment is comfortable, clean, with a lot of privacy and lays outside the busy centre of Kamari. For the fourth person a sofa could be used.
Nearby there is a very good bakery, a bus stop and an open air cinema which is really recommending. The beach is about 20 minutes by feet. Buses are driving a few times per hour to Fira. From Fira you can take buses to other places.
We had a great time at Santorini!