Often times, Demyx is to be found playing the role of uke, while Xigbar plays the role of seme.
Another, less commonly used term for this pair is 'Ximyx'.
(2) Fan 1: I like XigDem! :D
Fan 2: Whats that? :o
Fan 1: Thats the pairing of Xigbar and Demyx from Kingdom Hearts 2! :D