Jason is actually Charles! And I think they are twins!
I think Jason as we know him on the show is actually Charles and they switched places in Season 3!
I think something bad happened to or with Charles as they were young and he got locked up in radley. Something so bad, the family doesn’t even show him in their own house. You only see pictures of Alison and in their photo albums you can see only the 4 DiLaurentis’. They pretended that they have no other son.
The liars have to prepare a prom night from Melissas generation! This includes Jason, Ian, Garrett and obviously Charles! What have they in common? Right, the NAT-Club. -Besides of Charles, I think he wasn’t with them at this time. He might be locked up and couldn’t experience a High School Life nor prom night!-
Let’s recap the incidents that happened to Jason! Remember that someone got stabbed by Aria on the Halloween train in 3x13? Jason had a wound on his left hip fixed by Mona in 3x14!
Later on the show in 3x19 Jason had an incident with Emily in the elevator! After that, he was in the hospital and disappeared at the end of the episode!
Two episodes later in 3x21 Spencer found a dead body with Toby’s tattoo on and everyone thought it was Toby, but it wasn’t! Marlene said we will find out to who this dead body belonged to, so I think it has to be relevant! I think the dead body was Jason! The real Jason! Then I found this:
Interesting wound, don’t ya think? Yeah it’s the same with an fake tattoo on it! Great work Big A! I think Charles kidnapped Jason, his own brother, and killed him off! He switched places with him in Season 4 when “Jason” came back.
The most important quote what made me think of that is when “Jason” talked to Spencer during going through some family photo albums and said “Ali was always her -Mrs.D.’s- favorite”! And in the last episode “Welcome to the dollhouse” Mona as “Alison” received a gift from -A with the note “Because you’re my favorite”!