Sergei has given up on his last wish and it shows how he is a person who is prone to care more about other people than himself, even if he takes a long time to realize it.
Although you have not shown the text that this question refers to, we can tell from the context of your question that you are referring to "What, of This Goldfish, Would You Wish?"
With the reading of "What, of This Goldfish, Would You Wish?" we can see that:
Thus, we can see, when making the last wish Sergei gives up his own will and loses the company of the only friend he had.
This shows that even reluctant, Sergei is a person who tends to neglect his desires so that he can improve someone's life.
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