OMG! WT*?! It looks like Byakuya works with everyone from Scrubs?! How did this happen...?
Anyways, the thing is, demons can suck humans energy through a kiss and that which comes after that but when a human belongs to a demon, no other can use it. Well, that is only as long as the first demon is still alive or cares enough for the human. Fubuki, who doesn´t remember being the one for Byakuya, loses energy while staying in the human realm with the others. Realizing that no one hits on Byakuya he steals some kisses while he´s sleeping prepared to fight the "owner" who never appears. This time around, he has fought the whole day so he lacks a lot energy that´s wwhy he wakes Byakuya to get more reserves.
Byakuya: What are you doing?! You know that I´m not free!
Fubuki: Well, I´m prepared if he dares to show up now. *kisses him*
Byakuya, struggling: How many times have you snuck in here already?!
Fubuki, chuckling and continuing: More than enough to know he´s either dead or doesn´t care.
Byakuya is perplexed and doesn´t really retaliate (and I guess he doesn´t really want to given he really loves him deeply).