WMG / Lady of the Lake

Galahad was a secret stab at all the fans who said Ciri was a Canon Sue.
Sapkowski decided to show them the difference between a well-written powerful character and a real Marty Stu who gets everything without struggle. That was the whole point of the Gainax Ending.

Galahad was Sapkowski's own subtle way of acknowledging Ciri's status as a Canon Sue.
The book ends with the aforementioned characters riding side by side, holding hands, seemingly having entered a 'connection' of sorts. It's almost as if the author wants to say, "My very own Mary Sue meets the original Marty Stu. Oh, how perfect they look together!".

http://tvtropes.org/pmwiki/pmwiki.php/WMG/LadyOfTheLake