According to Swami Vivekananda, the father and mother give us the body; but the Guru gives us rebirth in the soul. In other words, the Guru is the person who has actually realized the Divine truth or has perceived himself or herself as the spirit.
Guru-disciple relationship is based on the genuineness of the guru and the way for subtle or advanced knowledge to be conveyed which has been attained by the guru himself or herself. The disciple through hard work and devotion eventually masters the knowledge that the guru embodies.
Mohindra in his blog writes that a true guru remains calm and serene at all times. He or she does not contradict or blatantly deny scientific theories, but subtly suggests limitations of science and scientific approach. Guru always focuses on abstract aspects of the ultimate aim of life or self-realization along with unfolding the the learning potential of disciples in science, art, intrinsic values and ethical ways of living.
Here is my ABC verse on a true guru.
A: Abstinence from non-virtuous
actions of body, senses and mind to gain and retain higher wisdom
B: Benign and gracious toward disciples or learners in protecting their self-worth
D: Disciplined as a result of mastery over self and knowledge with enthusiastic perseverance and joy of sharing knowledge
C: Compassionate toward all sentient beings and helpful in alleviating their sufferings
E: Even-mindedness towards all disciples or learners to make learning inclusive and equitable