Why does character of identity element always positive integer?

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I'm a chemist who is interested in mathematics. One of the subject that I'm interested in is group theory and representation theory, because of its application in chemistry.

I was looking at character tables and realize that the characters of identity element is always a positive integer (i.e. in $Sym(3)$ the character for identity element is 1 and 2, in $Sym(4)$ the character for identity element is 1, 2, and 3, etc.).

Is there any reason for why this is true?
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The character of the identity element is the trace of the identity matrix, i.e. the dimension (the "degree") of the representation.