Not Grover.
Okay so I read like almost everywhere that Grover was the first Percabeth shipper and I would have agreed with that except for one little part in the lighting thief. Grover wasn't the first Percabeth shipper, he was the second person to ship Percabeth.
Anyway the first Percabeth shipper has a lot of eyes
And his name starts with an A
It
Is
Argus
Let that sink in. The truth.