Sing-Sing is Cassima's pet nightingale. It carried Alexander's ring, love poetry, and white rose to Cassima, letting her know that Alexander was on the islands.
If you use it to send Cassima the items, but don't talk to Jollo about it first, you will still get Cassima's return items, but will not see the cutscenes. Alexander will be in the dark as to where the items came from.