Malory mentions this knight at the beginning of the war against the Emperor Lucius, calling him Ywaine's son. A few chapters earlier, Ywaine himself appears quite young. This indicates either that Malory did not arrange his material in strict chronology or that Ider was the sort of some other Ywaine, not Ywain, son of Uriens and Morgan.
Malory seems not to mention Ider again, unless he is identical with "Sir Idrus the good knight", who seems likewise to be mentioned but once. Ider and Idrus were on Arthur's side, but I find no firm evidence that he or they were members of the Round Table.