The is no real reason behind a character item. Back when Vocaloid 2 was growing in popularity the items were simple giving to characters as a running gag in videos that had become popular. For instance Miku got her spring onion from the leek spin meme due to a NicoVideo user editing a gif of her holding the spring onion instead of the character Orihime from Bleach. Others items are given based on the personality the fans have given a Vocaloid. For instance Prima character item is said to be a wine bottle because described as an Italian opera singer but also had a knife as her character item because of when people tried to download her illegally she would make it so that all Vocaloids would no longer work making it so " she killed them and her self ". Another one is Big Al and a pocket watch because of the fact that Al was released about 3 years behind schedule. However in latter years it has become not as much of a big deal for a Vocaloid to have an item.