He discovered Batman's secret identityand then drugs Bruce Wayne and usurps the role for a time being.
When Bruce Wayne's back is broken by Bane during the Knightfall storyline, Jean-Paul Valley, better known as Azrael, replaced him until he was deemed a poor replacement for Bruce Wayne and was subsequently replaced by Dick Grayson.
Best known as Batman's teen sidekick Robin and as his independent, adult persona Nightwing, Grayson became Batman twice. The first time was during Knightfall when Valley proved an unsuitable replacement and the second time was following the apparent death of Wayne in the Batman R.I.P. storyline. During the Battle for the Cowl storyline which followed Batman R.I.P. other characters briefly assumed the role, Jason Todd and Tim Drake, before the alias finally passed to Grayson. Grayson has also donned the costume on least one other occasion, although it was merely for one issue.
*Bonus fist pump with Damian!*
The third Robin. Tim Drake briefly assumed the role of Batman before "Batman: Battle for the Cowl", and then again during the event itself. He replaces Bruce Wayne in some continuities, notably the Titans Tomorrow continuity. He has also assumed the role in an apocalyptic future, after the "future's end" storyline.
The second Robin. Jason Todd usurped the identity of Batman during "Battle for the Cowl", but was defeated by Dick Grayson, who became Batman following. He is the second Batman on Earth-15.
The 5th and current Robin. Portrayed as the successor to Bruce Wayne and Dick Grayson in a possible future of mainstream continuity
The second Batman in the DCAU/Earth-12 universe and the fourth Batman in a possible future of mainstream continuity (prior to Flashpoint).
ollowing Bruce Wayne's apparent death in Batman #40 Gordon took up the mantle in Divergence #1 (both issues being part of The New 52).