This page contains spoilers. If you do not wish to be spoiled, do not read further.
When Princess Eilonwy went missing after coming to Mona to learn the ways of the court, Rhuddlum admitted to Taran that he knew Prince Rhun was inept, and asked Taran to pledge to keep him safe during the search. Taran found keeping this oath a challenge, but ultimately succeeded in this task.
Rhuddlum was always delighted when Kaw said his name.