Dr. Arvin Fennerman was a professor of the university that Alex was in, called California Institute of Technology (also known as Caltech). He was also Haley Marshall's ex-boyfriend and Alex Dunphy's current boyfriend.
Arvin first appeared in "In Your Head" when Haley snuck into Alex's class to get her to read over her writing sample before she went to an interview at Nerp. Haley ended up interrupting the class before Alex rushed her out. Arvin later found both of them in the school yard and told Haley that the two of them would get married one day, that he didn't understand it, but he just knew because all the probability scenes he had been running in his head ended up with the two of them marrying. Haley later used a similar speech to get the job she was applying for and later ended up accepting his contact request. It was unclear whether Arvin actually believed this or if it was just a clever pick-up line.
Arvin was a two-time Vanderscoff Grant winner: one for philosophy and one for geometry.
In the series finale, he and Alex got in a relationship.