Sure, they're selling Xboxes off Halo but I don't think it's always based off the character alone (ODST, Halo Wars, Reach). I don't think a mascot can come from an M-rated game either.
If you look at Nintendo, they've got Mario all over the place and in branding. Sony and MS don't really have this. Lately, it seems MS are pushing their 'avatars'.