Usually, in the Torah, the name of the Parsha comes from the first few words in the first possuk.
For example, the first Parsha in the Torah is called "Bereishis" and it comes from the first word in the first possuk there:

Another example is the next parsha of Noach, where the name is taken from the third word in the first possuk:

However, this is not so when it comes to the books known as Nach (Neviyim and Kesuvim). There it seems to be that the main character is chosen as the name. For example, Sefer Yehoshua is named after the new leader and all the happenings under him. Sefer Shoftim is about the leaders that arose to save the Jews from cruel rulers.

With regards to Megillas Esther, this chose of naming is open to discussion. This is because we know that there are other main characters in this story - King Achashverosh, Homon, Mordechai. Although there could be grounds to call it after the enemies, like is found by Megillas Antiyochus (by the Chanuka story), however, this is not synonymous with any naming of the other books in Nach. This leaves us with the names of the two saviours - Mordechai and Esther. Why was it named after Esther as opposed to “Megillas Mordechai”?

The Yalkut Meam Loez (1:1) provides three answers:

1) from the Yaaros Devash - aside from the name of the character, there is also a message that encapsulates the whole Megilla. This is seen in the name אסתר, Esther, that can be linked to the root סתר, Saser, meaning “to hide” or “to conceal”. The Megilla speaks of the miracles that were concealed and hidden.

2) There is nothing unusual in the events of the Megilla since they can all be understood in a natural manner. The exception is Esther’s marriage to Achashverosh. She was one of the seven prophetesses (Gemora Megilla 14a) and spiritually great. The fact she would marry a gentile was highly unusual. No one could understand it - until it was finally revealed that this marriage had been directed from Hashem to use Esther as a messenger well positioned in the palace to save the Jews from Homon’s plot.

3) it was Esther who advised the Jews to do teshuva by fasting for three days and nights (Esther 4:16) that was the catalyst that saved the Jews from annihilation.

Here is just an insight dealing with the naming of the Megilla - why specifically "Megillas Esther" that is brought in my introduction. However, there is much more that I discuss in my new Sefer called Megillas Esther Explained (Mosaica 2026-7) where I go through each passuk by passuk explaining how the it all flows together into an exciting and fascinating understanding of this well known story all using authentic sources.