In the Riftwar series there was a Tsurani who was trapped on the side of Midkemia, and who had to adapt to life on the Midkemian side.
He became close to the pivotal characters, eventually helping to interpret Tsurani customs and such, and to help set up the peace talks I believe.
Not sure if Borric was still alive at that point, but the character was close at least to Borric's sons.
All I know is that his name vaguely resembled my name (I based my name on his), but I forgot what his original name is, and I can't find it back, all I find is the surrounding characters.
I might have blended characters together, it has been 10 years since I read those books for the last time.
It isn't Kasumi but jontia's answer did spark some more memory, it was more a normal soldier/warrior who was a prisoner of war at first, didn't speak Midkemian at all, and had to learn it.
Does anyone know who I mean, or do I have to read the books again to stumble upon him again?