Yep! Yue is really Yukito! I know most every one already knows this, but I didn't want to spoil it for someone who didn't.
When Sakura catches the last Clow Card, Yukito passes out and Yue appears! This is the reason that Yukito has to eat so much but never gets full. It takes a tremendous amount of energy to support Yukito's form. Once Sakura catches all the cards, Yue is awakened!
Once Sakura becomes Mistress
of the Clow Cards, Yue appears quite often to help her. When he does
begin to appear regularly, Yukito is unaware of what is really going on,
and is always confused about how he loses so much time without noticing.
Eventually, Sakura's limited power becomes insufficient to support both
Yukito and Yue's forms. Yukito's personality and body will have to
be sacrificed unless something can be done. Touya isn't willing to
lose Yukito, so he donates all his magic power to Yue, so that he will
be able to maintain himself and Yukito. Touya makes Yue promise that
he will protect Sakura in his place, now that he no longer has any special
ability to take care of her himself.