I like old songs:
The nightingale by Julee Cruise for: Feilong, Yoh and Tao
https://youtu.be/c8MLRVsoE8w
Your Love Is The Greatest Gift Of All by Michelle Wright for: Asami and Takaba.
Just thought of one for Sakazaki - "I 'm Too Sexy" https://www.youtube.com/watch?v=P5mtclwloEQ - maybe
Not for a a specific character, but you just have to throw Marvin Gaye's "Sexual Healing" in there somewhere. ;) Not the exact lyrics as such, but the song and the sensual vibe...
ASAMI: At first I thought of Salt N Pepa feat En Vogue - "Whatta Man"? *grins* Just kidding. No, the song I would choose for him is by French singer Guesch Partie - "Etienne". That song was hot as f... The English translation is incredibly tame, the french lyrics are so much more erotic. But you will get the general idea... ;)
http://lyricstranslate.com/en/etienne-etienne.html-0
AKIHITO: Considering Akihito's strength and resiliance and given all that he's been through, maybe Destiny's Child - "Survivor".
Btw, Sara Bareilles - "Gravity" might have been a nice choice for that time when Akihito tried to run away from Asami, as it deals with the complexity of relationships. Again, not the exact words so much and not for their relationship in general, but the vibe of the song seemed to go along with Akihito's downcast mood at the time. Plus, it's a gorgeous song. :)
https://play.google.com/music/preview/Tbjvtaoznly3uqxfwc4w3ffa7qe?lyrics=1&utm_source=google&utm_medium=search&utm_campaign=lyrics&pcampaignid=kp-lyrics
I like Musicals, so I thought Unusual Way from Nine would fit Feilong...and then there's Goldeneye. And speaking of James Bond songs, Nobody does it better wouldn't be too bad a fit for Akihito.
As for Asami...
https://www.youtube.com/watch?v=LzaSBzs8mYE
So if you had to pick a theme song for your favorite side character, what would it be?
I would pick Spy in the House of Love by Was (Not Was) for Kuroda.
https://www.youtube.com/watch?v=k8PjjxJ7v78