WARNING! This wiki page contains major spoilers!Edit
Dexter Tempest is Grace and Connor's father (but not the twins' biological father, who is revealed to be Sidorio in the fifth book) Dexter met their mother while she was present on The Nocturne as a donor to the soon-to-be renegade Sidorio. At some point Sally and her friends spotted a man, who appeared to be Dexter, on the nearby shore and her friends dared her to swim over to him and say hello. She swam over to say hello and they ate his lunch together. He wanted to come back with her but she persisted in claiming that The Nocturne was 'a private and unusual ship' and he said he would find a way to meet her again.
A man of his word, Dexter kept his promise and soon appled to be the chef aboard The Nocturne and the two lovers-to-be met again and Sally informed The Captain (soon to be revealed as Sidorio's undercover deputy, Obsidian Darke) that she had to go with him. This was not to be. Sidorio apparently had in-depth feelings for Sally and, out of sheer jealousy and anger for Dexter, he wished to leave a lasting mark on the poor girl. So, Sidorio employed the dark arts of a medicine woman to cast a 'pregnancy spell' on the unwilling mother as she slept. Unbeknownst to the twins, this in turn made them both dhampirs (extremely powerful half-mortal, half-vampire beings) as the ritual executed by the medicine woman required a drop of his blood, thus imprinting the unborn twins with the Vampirate's DNA. Sally, Dexter and The Captain soon discovered this dark secret and the Captain eventually found out the unborn children were of Sidorio's blood. To free Sally from further torment, the Captain took the burden of these troubling memories (those being of knowing that Sidorio was the twins' father). This is why Sally's soul appears during the Captain's healing process.
At some point, though, Sally must have come to a grisly end at the hands of Sidorio, grief-stricken and mad with anger. Following this, Dexter took Connor and Grace to the small, unmarked coastal town of Crescent Moon Bay, and, several years later, the start of the first Vampirates book takes place. Dexter dies after one or two chapters. Dexter Tempest is not in the official site's Vampirates: 'Meet the Crew' section.