I have removed Google Now in order to assign long press on home button to open another app. But recently it comes back with a different interface I'm familiar with. I guess the phone just has updated it.
However I'm unable to disable or uninstall it. In Google Play I can't find which app has just been updated. In the setting of the app there is no option to disable it.
These are some system Google apps that's under my suspicion: Google Services Framework, Google Partner Setup, Google One Time Init, Google Play Store and Google Play services. But I find no evidence to against them.