From the FW Blog, Nov. 6, 2012:
At the moment, I’m hard at work on what is turning out to be a bit of a coda to Lisa’s Story. Events in the present will spark a sort of flashback/prequel which will crossover into real life with a visit to my old apartment in Elyria…as well as a crossover with Crankshaft thrown in for good measure. Oh, and lest I forget, a long lost character as well…Stay tuned.

Clearly Les does not share Cayla’s excitement over his basic-cable movie getting a free plug on a basic-cable news network. Meanwhile, somewhere in Elyria, Ohio, “a long lost character” with Paulie Walnuts hair and wearing a wifebeater clutches a can of beer and talks to his TV. It can be none other than the despicable Frankie, Darin’s biological father and St. Lisa’s…ex-boyfriend? Date-rapist?
Tom Batiuk Talks ‘Funky Winkerbean’, comicbookresources.com, March 19, 2013:
I ended up writing a story where Frankie — he’s been mentioned a couple times and has actually appeared in the strip very briefly, the guy who got Lisa pregnant — returns. In the return of that story we deepen the teen pregnancy story and say that it was a little more than just youthful indiscretion on Lisa’s part. There was some coercion involved and it’s like a coda to “Lisa’s Story.”
Guess we’ll have to wait for TB to “roll out” the story to see how he retcons this.

I wasn’t able to dig up a whole lot on the web about Frankie (he’s not even mentioned on the Unofficial FW Fan Page, though Lisa’s teen pregnancy is). I found the above strip from Act II, but can’t provide any context on what their relationship was by the time she was pregnant with Summer.