22 SEGA racers
1 DLC SEGA racer
2 guests (Ralph, Danica)
2 avatars (Mii, Avatar)
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Total: 27
Like you said they have 29 characters total, so either the 2 remaining are SEGA characters or they are guests unique to the XBOX and Playstation platforms. I'm hoping for the latter, I don't want SEGA characters to be exclusive to platforms. Ideally, everybody gets to play as the SEGA racers and guests/avatars are exclusive to platforms.
Also, 17 total SEGA characters are known of at this point. So 6 remain, and Steve said that at least 2 will not be announced until the game launches and people learn of them by playing. So... 4 remain to be announced. Also, Steve implied that there will be more doubling up for IPs. So we might see another Skies of Arcadia or Monkey Ball character. Or two of those might be Space Channel 5.
My final six guess is:
Alex Kidd
Aika
Ulala
MeeMee or Pudding or Billy Hatcher - some character that will piss people off
Ryo Hazuki
Segata Sanshiro
Flagman:
Opa Opa