Rohan first meets Suko in episode 2, and they are rivals through much of the series.
Later, it becomes difficult for Rohan because his parents become friends with Suba's parents. They are expected to be nice to each other. It is ultimately revealed that Suba is somewhat neurotic and has less self-esteem than is assumed by the end of series 1, and a lot of high expectations placed on him (mainly by his parents), which further depresses him. At his 10th birthday celebration in Sapporo Park, Rohan (and his best friend by that point, Dewey), realize the extent of this, and determine that by helping Suba (who is also present, but not enjoying himself) realize that they can all be friends, despite their differences, is the best approach.
In Series 2, Rohan and Suba's relationship is more of a friendly rivalry.