Both Hardin and Pirenne want the same thing, for Foundation to fulfill its purpose for the greater good of the universe. But, and this is a big but, they don't agree on what the good of the universe is. Pirenne sees the completion of the Encyclopedia Galactica as the best service to end the inevitable Dark Age, while Hardin believes they should actually use that knowledge to their advantage in deals with the Four Kingdoms.
So why isn't Pirenne an antagonist? The way each character views Foundation's purpose highlights the different characteristics of the other character. They're foils, not enemies.
Ponyets and Gorov are foils like Hardin and Pirenne. Each believes trading technology with the other worlds is important, but they have distinctive ways of accomplishing the goal.
Gorov thinks that what the Foundation needs to do is sell the importance of the machine. In other words, the other worlds must see the value in the technology itself, not just what the technology will do for them (IV.6.10).
Ponyets just wants to make the trade, and it seems like he'll use any means necessary to do so (he resorts to blackmail with Phrel). For Ponyets, so long as the technology gets out there, the Foundation will have the upper-hand.