It is certainly very good practice to match data types, but that is not the only way to obtain a seek here (assuming the default collation is a Windows one like Latin1_General_CI_AS rather than a SQL one like SQL_Latin1_General_CP1_CI_AS):
CREATE PROC QOD_Performance_1
@Get NVARCHAR(20)
AS
SELECT ID,Col
FROM FirstTable WITH (FORCESEEK)
WHERE Col = @Get;
GO
EXECUTE dbo.QOD_Performance_1
@Get = N'XYZ'
Wondering how this works, or what the Constant Scan and Compute Scalar are for? I wrote about the hard work put in by the optimizer with these sorts of data type mismatches here:
Paul White
SQLPerformance.com
SQLkiwi blog
@SQL_Kiwi