Final answer:
The author might use the goldfish archetype to develop the events of the story and Sergei's character by introducing a sense of mystery and wonder, and as a catalyst for the plot.
Step-by-step explanation:
An archetype is an original model for a type of character. In the story, the goldfish is an example of a magical being archetype. The author might use this archetype to develop the events of the story and Sergei's character in order to introduce a sense of mystery and wonder. By including a magical being archetype like the goldfish, the author can create a unique and fantastical world that engages the reader's imagination. The goldfish's presence can also serve as a catalyst for the plot, leading to important events and character development.