What qualities does a creature born in the North have? Why?
You must be referring to the Prince of Morocco's speech in Act 2, Scene 1, in which he says,
Bring me the fairest creature northward born,
Where Phoebus' fire scarce thaws the icicles,
And let us make incision for your love
To prove whose blood is reddest, his or mine.
In this scene, the Prince of Morocco has come to woo Portia. He comes knowing that he is at a disadvantage among her suitors, because he is dark-skinned. This reflects the value, sadly held at the time, that dark skin is less attractive than light skin.
Thus the first words out of the Prince's mouth are to ask Portia not to pre-judge him because of his "complexion,/The shadowed livery of the burnished sun." Then he speaks the lines above, pointing out that his blood is as red as any fair-skinned suitor's. He later goes on to say that he is considered attractive in his home country.
So, the quality of a "creature northward born" which the Prince is addressing, is that of being fair-skinned.
Not coincidentally, "fair" in Shakespearean English could also mean beautiful, as in "fair weather."
So the Prince is invoking the idea of a rival suitor who is extremely "fair" (i.e. handsome), but also very fair-skinned, coming from a place where the sun's fire can scarcely melt the icicles. This conjures up a mental image of someone who is very, very pale, as if after a long winter.