Who is this guy helping Harry Osborn?

That man is Gustav Fiers, also known as the "Man in the Shadows". You may remember him from the first film:

Here he visited Curt Connors to ask whether he had told Peter the truth about his father. When Curt tells him to leave Peter alone, he disappears.

In that film, he is credited as the Man in the Shadows. In Amazing Spiderman 2 he is finally given a name, being credited as "Gustav Fiers (The Gentleman)".

A quick search for Gustav Fiers reveals:

During Adam-Troy Castro's Sinister Six trilogy of Spider-Man novels, a new Sinister Six - consisting of Doctor Octopus, the Vulture, Electro, Mysterio, the Chameleon, and an original character called Pity - are formed by Gustav Fiers, a notorious criminal known as the Gentleman, who wishes to kill Peter Parker in revenge for the actions of his parents in thwarting some of the Gentleman's schemes in the 1960s.

It appears likely therefore that he will be some sort of "super villain" in the remaining movie(s).