Peter is a Meyerpire that used to work with Jasper under Maria during the Great Southern Vampire Wars (GSVW.) Charlotte is his mate, and has no characterization beyond that. Peter eventually got tired of the GSVW, took Charlotte, and left with Jaspers help. Jasper eventually joined them, making Maria angry at Peter, then left them when his own twu luv! showed up. They still keep in touch.
Charlotte and Peter are
real vampires non-vegetarian vampires sparklepires (meaning they hunt humans), who visit during Midnight Sun. Edward can't spend time with them, because he is too consumed by Bella's smell (creepy). During their stay, the Cullens demonstrate strong commitment to their ethics, by (what else?) helping Charlotte and Peter hunt humans because they're so compassionate!