VGFacts

Although the Robin seen in Batman: Arkham City is Tim Drake, his Animated Series DLC costume depicts Dick Grayson's Robin.

http://vignette1.wikia.nocookie.net/batm...1028194619 (Ingame screenshot of Animated Series Robin)

http://dcau.wikia.com/wiki/Nightwing (The DCAU Wiki article on Nightwing/Dick Grayson, which also includes an imag eof his "Robin" persona from the animated series.)

http://vignette2.wikia.nocookie.net/dcan...1103002907 (Direct image of Dick Grayson-Robin from the animated series)

As a side note, Dick Grayson as Nightwing is a DLC character for the game as well, complete with animated series outfit.

To build on this, Dick's Robin suit was Red, Yellow and Green with a black cape, where as Tim's is Red, Yellow and Black, without any green.

Which is weird, as Tim was also in the DCAU and they could have easily used his suit instead of Dick's.

They probably used Dick as it was the best match for Tim's collision data, design and height wise