Tzaddi = 90, ordinal 18, reduced 9, final form ץ = 900. Spelled in full, צדי = 90+4+10 = 104.
Meaning
Tzaddi is read as 'righteous one' (tsaddik) and its form as a kneeling figure, which makes it the letter of the hidden saint and of the covenant; the older philological guess is 'fishhook', and the handbooks give both. Its 90 is the last of the tens and its final form 900 the last of the hundreds, so tzaddi closes both series at once.