IF OBJECT_ID(N'tempdb..#TC46') IS NOT NULL 
    DROP TABLE #TC46 ;
	
CREATE TABLE #TC46 (String VARCHAR(MAX)) ;

INSERT INTO
    #TC46 (String)
VALUES
    ('aaaaaAbbcad'),
    ('X8JXab'),
    ('999744499XYZ'),
    ('BBBBBBBBBBBBBBBA'),
    ('AAAAAAAAAAAAAAAA') ;

SELECT
    ISNULL(NULLIF(Stripped, String), SUBSTRING(String, 1, 1))
FROM
    #TC46
    CROSS APPLY (SELECT
                    SUBSTRING(String,
                              PATINDEX('%[^' + SUBSTRING(String COLLATE SQL_Latin1_General_CP1_CS_AS,
                                                         1, 1) + ']%', String),
                              LEN(String) + 1) AS Stripped) AS LeadStripper ;