probably using an index/match.
something like this...=INDEX('Helper List'!$A$2:$A$1001,MATCH(B2,'Helper List'!$B$2:$B$1001,0)) dragged down.
the misses will show as #N/A so you can add an iferror to it to show mismatches like this...
=IFERROR(INDEX('Helper List'!$A$2:$A$1001,MATCH(B2,'Helper List'!$B$2:$B$1001,0)),"missing")