Most people just think plain saying that Nocturnal is the enemy of the Fox since he stole her cowl....
But I think she chose him in some sort off way that he is her champion...
First reason is that she allowed him to keep the cowl, she cursed it, but he could keep it, if she really wanted it back she would have used more force, mayby even sent some people after it (see did it to retrieve her eye remember)... A daedra would easily get back an item from a thief... The cowl itself couldn't be really used against her since the curse is the thing that makes it so powerfull.... wich brings me to my second reason:
The second reason why I think that The fox is more or less her champion is because of the curse itself... In many ways it's really a curse to the person since he isn't able to feel the warm love of his family and friends again... On the other hand it makes him a hell of a thief... Even better then a ungoing invisibility spell... Being able to take it off any time without ever being recogenized (sorry for the english ).. the life detection and the bg feather spell allows him to carry awfull lot of loot...
The Third reason, wich is actually more of a conclusion of the others is that it just all fits within Nocturnal's personality.. since the fox will be hidden for ever not being revealed to the daylight ever....
The Fourth reason, a quite short one.. is that almost every thieves guild members say at the end: "shadow hide you" wich is written on the cowl... so in some ways "shadow hide you" is a blessing of Nocturnal to the wearer of the cowl...
The last reason, wich might be wrong since I don't know really much about the power of the Elderscrolls and such but..... When the curse was lifted.... That would mean, that if Nocturnal is the fox' enemy, she would curse it again, or i that's impossible another curse, or just tries to retrieve it.... but... first off all, the whole curse isn't lifted, the positive site of the curse, the site wich makes the fox a master thief, still remains on the cowl, so you could say that in some ways she allows the Fox to be the master thief.....
So from this reasons I got the conclusions that the fox is more or less Nocturnal's Champion because she chose to after the cowl was stolen from her.. (who knows, mayby even that was somekind off set-up).... but let's not discuss that....