This bottle of cherry flavoured syrup can be found in Citadel Island's pharmacy.
To convince the rabibunny that works at the Principal Island's library to show him the book about the Legend, Twinsen must do something about the bad flavour of the water of the island. To do that, he must get the bottle of syrup and pour it in the water tank in the water tower.
After using the syrup, Twinsen keeps the empty bottle.
"Bottle of Syrup. This bottle contains 50cl of very concentrated red syrup."