Who is Rabbi Issac Luria also known as the ARI?

Simply put! The ARI - Rabbi Issac Luria - is the best known Kabbalist since the writing of the Zohar. He developed an understanding of the Kabbalah called the Lurianic system.

All Kabbalists since his birth follow his system. There is no exceptions. It is for this reason that his recommendations are taken to heart by almost all teachers of the Kabbalah

He also teaches that the last 9days of the 3 week period are the toughest and he chose to leave this world on the 5th day of the 5th month. Hay B'Av so that we can utilize his essence to help balance the period.