So let's just suppose you've put Giles into a relationship with somebody. Is this person going to call him "Giles" at intimate moments? Probably not. And yet, the urge to go with something other than the obvious "Rupert" is tempting.
Amongst the nicknames for Rupert Giles are such names as:
Rupert. Don't get cute with this.
Rupe. One syllable, for the modern lifestyle.
Rupes. Because Spike is our role model.
Ru. We went and got cute.
Ripper. Simple, classic, canonical.
Rip. Two syllables are too many.
G-man. Everybody say it together...
Giles. Because he's like Sting.
Something clever you will tell us in a comment.