Percy was forgotten by all the Greek campers after he left camp half blood due to some...um ...relationship problems.
When Percy leaves he is blessed by Hephaestus so that he can use mortal technology, and Hephaestus tells him about who his first step father really was.
He will take over the company and billions in his name and carry on his first stepfathers work.
He does have the heart thingy.
Takes place after the Avengers but as if age of Ultron happened but didn't at the same time.
(AKA the characters are there and everything that happened after it is there but the actual event will never be mentioned.)