Ok for this Secret achievement you have to do the following:Set both Free Play Characters as The Joker and The Joker Animated Serirs
Do do this you first need to play and complete the dlc story mission, then you will unlock 8 Characters including The Joker Animated.
These are as follows:Batman (B:TAS)
The Joker (B:TAS)
Harley Quinn (B:TAS)
The Mad Hatter
Now this is done if you load up the game or quit to Gotham City Free Play Roam and do the following:The Joker Animated (B:TAS)
Press and hold
to bring up your Character select screen, scroll to row 9 and go across to character tile 20 and select with
to change to your second free roam character The Joker
Now press and hold
once again to open up character select screen, scroll to row 4 and go across to character tile 10 and select with
As soon as you have done both pop achievement unlocked!