What is the equivalent of the SQL Server CHAR function?
Have a look at the docs on PostgreSQL String Functions and Operators,


  chr(int) Character with the given code. For UTF8 the argument is treated as a Unicode code point. For other multibyte encodings the argument must designate an ASCII character. The NULL (0) character is not allowed because text data types cannot store such bytes.  


Demonstrating chr(), its inverse, and both.,

# SELECT chr(65), ascii('A'), chr(ascii('A'));
 chr | ascii | chr 
-----+-------+-----
 A   |    65 | A
(1 row)


In your question you use E'' (the "bytea Hex Format") which is totally possible too, though I wouldn't suggest it for simple problems like this. You can do that like this,

SELECT E'\x41'; -- 41 is hex for 65 (in base 10)
SELECT E'\101'; -- 101 is octal for 65 (in base 10)
 ?column? 
----------
 A
(1 row)