I was staying with a friend at begoñas place and she was really nice and even managed to get us an extra mattress so we both had our own bed. We never met her actually but her brother took very good care of us and made sure we were comfortable and found our way around.
The flat is not so close to the city centre, i took us an hour walking but busses are faster.
We really recommend staying with Begoña. The kitchen is really nice and we were also allowed to use the washing machine. So go there and enjoy Sevilla:)