Sister Jenna is a character Roland Deschain encounters in the novella, The Little Sisters of Eluria (and the comics by the same name). Over the course of the novella, Jenna falls in love with Roland and helps him escape, betraying her sisters in the process. Roland also becomes her lover (briefly) at the end of the comics.
She was damned by the Little Sisters and such her body dissolved into Cam Tam.
She is a vampire, but unlike her sisters who use glammer to appear younger, Jenna's youthfulness is real.