Character Counting System
MATL, 3 bytes
8#u

Input is a string enclosed with quotation marks (which is allowed by default)
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Explanation
u is the unique function, which essentially removes duplicates. It can produce up to four outputs. The fourth output is the count of unique characters, identified in order of appearance.
8# specifies that the fourth output of u should be produced. In general, 2# means &quot;produce two outputs&quot;, 3# means &quot;produce three outputs&quot; etc. When the maximum number of function outputs is reached, larger numbers mean &quot;take the first output only&quot;, or &quot;the second only&quot; etc. Thus, since u has four possible outputs, 4# would produce all four outputs; 5# produces the first, 6# the second, etc.