The real story: I created Stephen Kowalski to follow Hadley. Since it's so Polish-American, Hadley could tell him, "Mama told me that the Poles are the French of the Slavic world.” It's also a running joke that Hadley's mom doesn't like any ethnicity (Germans, Hungarians, Austrians, and Russians because she's Slovak and they all screwed over the Slovaks at some point or another).
Hadley sparks life inside of him so to show this intimacy he shares his actual name with her. The real name had to be Polish so she could say her mother would still hate him. I found the most gloriously confusing and blatantly Polish name I could find: Szczepanski. It looks so scary and foreign that the reader couldn't possibly confuse it with any of the other Slavic names I use.
It works on another level based on http://freepages.genealogy.rootsweb.ancestry.com/~jkmacmul/namemeanings-coatofarms/polishsurnames.html because centuries ago noble family clan names would be based on the name of the village they owned with -ski added to prove their nobility. I looked for towns starting with Szczepan- on maps and all the towns I found were all located on the southern boarder of Poland (CZ, SK and UK) roughly 100 Km from Krakow in both directions - most likely proving that Szczepan is a local diminutive of Stephon. So I knew where he had to be from and I really liked that he was Stephon of the town Little Stephon.
Kowalski is an entirely different matter. A kowal is a blacksmith, the lower social class. In the 19th century the commoners started adding -ski to their last names to make them sound more highfalutin, more noble. So it's kind of like the Superman/Clark Kent thing. His true self is noble and the guise he took in America is working class, normal, and false.
In preparation for answering this I've been reading over some of the stuff I wrote about GEM. When Stephon arrived in America he communicated through Jack Kowalski's mother, aunt to his real father, Bronisław Szczepański. Grandma Kowalski simply married a commoner and that's how it all started oh so many years ago in Southern Poland.