Hikaru Utada's a tough one to duplicate... but these artists might be to your liking...
Ayumi Hamasaki is a bit of a rival to Hikki, since they both came into popularity at around the same time... her music's much more pop-oriented and less R&B-ish. Check out her stuff from "Duty" and "I am..." first: maybe "Vogue", "Never Ever", "Seasons", "evolution", etc.
Namie Amuro is the precursor for both Ayumi and Hikki, and incorporates some of the same influences as Hikki... just distilled through the TK filter. Check out some older hits like "How to Be a Girl", "Dreaming I was dreaming", "Respect the Power of Love" and "Love 2000" among others. You might also want to check her mor recent work with the Suite Chic combo...
Mai Kuraki's another one worth looking into... she originally looked like she woudl be Hikki's rival before Ayumi's popularity skyrocketed. Check out her first works like "Secret of My Heart" and "Never Gonna Give You Up" before moving into her new works...
Yuki Koyanagi's another good contemporary... "Aijou" is a good song to start.
You might also want to try some neo-soul artists like Soulhead, or delve into some of Mondo Grosso's great work... "Life" featuring singer bird is one of my personal favorites...
Hope these suggestions help... as for Pillows, I'm not as familiar with their style. Any guidance here? I could possibly name some groups then...
*whew* I'll be more brief next time...