>>55623344It's based on how the numbers are read. Mi is 3 because 3 can be read as "mittsu", the zu in Mizuki is 2 because it's a voiced tsu and 2 can be read as "tsuu" (katakana spelling of the English "two") and ki should be 9 because 9 can be read as "kyuu", and "kyu" is ki+yu".
All kana have a number associated with them in this way, it allows for mnemonics, codes, special dates etc etc.
