Where in the book does it say Min Wife is a nice person?

I think this is done through indirect character representation. That is to say we find out about Min by the thgings he says and does rather than the author telling us. Min's wife comes out and gives Tree-ear his midday meal each day and is also the one who gives Tree-ear warm clothes when winter approaches. In time, Tree-ear and Min's wife develop a close relationship. In fact, Min's wife encourages Tree-ear to call her Ajima, a name that is like aunt and is reserved for highly respected adult family members. Tree-ear is deeply touched by this gesture. Later, when Tree-ear returns from Songdo, Ajima gives him the name Hyung-pil and asks him to live with her and Min. Page numbers differ depending on the publication of your book copy.