Secret lover? (9)
(Other definitions for confidant that I've seen before include "Sharer of secrets" , "Trusted friend" , "Close friend entrusted with secrets etc" , "intimate" .)
(Other definitions for paramour that I've seen before include "wife's toy boy, for example" , "Mistress, lover" , "courtesan" , "Lover, usually illicit" , "fancy woman" .)