Delta is the first fragment split from what he calls Alpha. At about 6, he is the only alter who actually got to meet Allison.

He is the last to choose a new name, and still ends up calling himself Delta a lot internally. The new name is Desmond.

While he can come off a bit emotionally removed, he cares deeply about preserving and taking care of not just themselves but also finds himself caring deeply for Caboose. Both parties struggle with memory, and Delta's memory is strong enough for the both of them.

But that doesn't mean he's always on top of his game, and being sufficiently warm when Church has been such a shut in the past while isn't something he's had to plan for.