Adi Shankaracharya on Guru in Bhajagovindam

Trust yourself wholly to the lotus feet of the Guru,
Freed from the shackles of samsara,
With your senses and mind controlled in this manner,
You will see God residing in your heart

Adi Shankaracharya

Please note: That which helps in realizing the Brahman in you is the real Guru. Guru can be a human or living or non-living being. It can be even a combination of any of these.