The bender is Roku's earthbending master. This is alluded to several times in the episode.
they are subtle but many.
1) when Roku says the training was bitter work.Iconcede it can just refer to kung fu being the translation of bitter work. but when he describes his masterthe description fit toph as well.
2) The bender in the opening was intended to be toph. Literally he was the first incarnation of toph.
3) The bender was toph in a previous life. Toph asks if friendships can span generations. This is a metaphorical representation of the original character design's intended use.
4) Zuko's inevitable friendship with aangmirrors this idea
just a theory.