Tzaddi (Tsadi) is the eighteenth Hebrew letter and a simple letter; its name is traditionally glossed 'fishhook' and its value is 90. It has a distinct final form and is linked by commentators to the tzaddik, the righteous one. The Sefer Yetzirah (Book of Formation), the foundational text of Jewish letter-mysticism compiled in late antiquity, divides the twenty-two Hebrew consonants into three groups: three 'mother' letters (Aleph, Mem, Shin) assigned to the elements Air, Water and Fire; seven 'double' letters, each with a hard and soft pronunciation, assigned to the seven classical planets and to seven pairs of opposites; and twelve 'simple' letters assigned to the twelve signs of the zodiac and the twelve months. As a simple letter Tzaddi is assigned to the sign Aquarius and the month of Shevat.
A separate and much later system, the Hermetic Order of the Golden Dawn (founded 1888), mapped the twenty-two letters onto the twenty-two trumps of the tarot along the paths of the Kabbalistic Tree of Life. This tarot correspondence is a modern Western-esoteric overlay, not part of the ancient Hebrew or classical Kabbalistic tradition.
The Golden Dawn originally assigned Tzaddi to The Star (trump XVII), a natural fit for the water-bearer Aquarius. Aleister Crowley's Thelemic revision (1904) declared 'Tzaddi is not the Star' and swapped the trumps of Tzaddi and Heh (keeping their signs), giving Tzaddi The Emperor in the Thoth system; the original Golden Dawn attribution remains The Star.