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.
Her mother had attempted to leave the Little Sisters and and break her vows, but returned and brought Jenna back with her and died shortly thereafter. Jenna is a lessor vampire, who has dined on human blood with the Little Sisters, but unlike her sisters who use glamour to appear younger, Jenna's youthfulness is real and she is able to briefly touch medallions with the sign of the cross. She also controls the Dark Bells - the bells used to call the Cam Tam.
She was damned by the Little Sisters and, as such, her body dissolved into Cam Tam.
She is the daughter of Alejandra and Bertrand Allgood.citation needed