Well, not sure about the Giratina set.
As for other members, yeah, go with Drapion. I say also go with Spiritomb as he does help symbolize the "Nocturne" theme.
On top of the theme, might I recommend using Darkrai instead of Giratina? He's one legend that definitely reminds me of "Nocturne" and he's also a Dark-type. His movepool is also pretty good too.
Darkrai
Dark Void
Dark Pulse
Calm Mind
Substitute/Taunt/Aerial Ace
If you go with Aerial Ace, give it a Rash nature. Other than that, stick with Modest obviously lol.
Hmm...not sure what else you should use. Maybe Gengar/Dusknoir, they're good Ghost Pokemon that also remind me of "Nocturne"
Good luck with that and hope it helps.