I tracked the family back to 1870.Stephen is living with his father Edward and mother Catherine.In that household, there is also a Metta Bullard and her daughter Florence.
I went back one more census to 1860 and found that Edward and Catherine had a daughter Marietta who was sister to your Stephen.So Bullard is actually related to Stephen's side of the family.
I believe that Agnes may be a Johnson.Using the same search, I noted that living right next door to the Gernon family was the Johnson family with a daughter Agnes of approximately the correct age.I also noted that both her parents were born in Scotland; something that Agnes Gernon reported during subsequent census years.Of course this should be verified, but it would make sense given the close proximity of the families.
With regard to where is Agness buried: In the 1900 census, Stephen and Agnes were living in Genesee twp. and Stephen owns his farm in this very rural area.In the 1905 Wisconsin State Census however, they are now living in the City of Waukesha and Stephen and Agnes are renters.Since Stephen died in 1906, I suspected that he was probably buried in one of the city cemeteries.The largest and oldest cemetery in the city is Prairie Home Cemetery. So I went there and found the whole family!