When you start a new game and select the 'Missions' option at the 'Main' menu enter UUDDLRLRBA as your name when prompted. Note: when this code has been enabled you cannot use any new character in Story mode, only in 'Missions' mode.
A 'Grip Gauge' will atomatically appear on screen whenever you are hanging from a ledge. If you do plenty of pull-ups which are performed by pressing L2 and R2 simultanously you can increase the 'Grip Gauge' which will then increase the amount of time that you are able to hang from a ledge.
Grip Gauge Level 2: Do 100 Pull-ups.
Grip Gauge Level 3: Do 100 Pull-ups after unlocking Grip Gauge Level 2.