The only one who saw her for

The only one who saw her for
 

p.53
-I was the only one who saw her for what she was.

SHE refers to Harry's witch mother. But what do the red words mean? My attempt is, FOR is like AS. But it doesn't help much.

to see someone for what he/she is = to know the true character of someone

The speaker was the one only one who knew the true character of Harry's mother.

Thank you.