I've chosen for nchar(1) because it only says that the value won't be NULL or empty, but there's no limitation on the characters used. Also depending on your collation sometimes a character is covered by char but in another collation it's a nchar.

Try this:

CREATE TABLE #Chars(

c1 char(1) COLLATE SQL_Latin1_General_Cp850_BIN,

c2 nchar(1)COLLATE SQL_Latin1_General_Cp850_BIN,

c3 char(1)COLLATE SQL_Latin1_General_CP1_CI_AS,

c4 nchar(1)COLLATE SQL_Latin1_General_CP1_CI_AS)

INSERT INTO #chars

VALUES(CHAR(0128),CHAR(0128),CHAR(0128),CHAR(0128))

SELECT * FROM #chars

DROP TABLE #chars

As you can see the nchar value is consistent, while the char value depends on the collation.