Hi eTess. If you are really worried about security then I suggest not using the facial recognition. As a note, Kobo did not develop the facial recognition software, but it comes bundled with Android.
The facial recognition is far from perfect, for example I can get my brother who a few years younger then me to unlock the my galaxy nexus by holding it an an angle.
The best form of security is the PIN or Pattern Lock.
As an added bonus, try turning on encryption. This will prevent any random person from reading the contents of your device if you are to ever loose it!