The Midrash states: "Hadassa" [Hebrew for "myrtle"] is [another name for] Esther. In the same way that myrtle has a sweet smell but a bitter taste, so was Esther sweet for Mordecai, yet bitter to Haman. (Esther Rabba 6:5)
...Esther sweet for Mordecai, yet bitter to Haman.
The "Chida" [a great rabbi known by the initials of his name, Rabbi Chaim Yosef David Azulai, of blessed memory] wrote: Esther was a reincarnation of Eve, the Serpent was enclothed in Haman, and Mordecai was the reincarnation of Jacob, as recorded by the students of the Ari. Thus, Esther (as Eve) was bitter to Haman, who was the Serpent, who had brought her to sin - and sweet to Mordecai, who, as a reincarnation of Jacob, was also the reincarnation of Adam. (Dvash L'pi 5:2)
And here I come to add, in accordance with what I've written on many an occasion, that every one of the [five] senses was recorded in the account of the sin of Adam - except that of the sense of smell. Therefore, understand that the sense of smell wasn't ever really blemished, and for this reason it is a thing which the soul [Neshama] gets pleasure from, as opposed to the body.
Thus, the scent was sweet to Mordecai. (And also because he himself is called "pure myrrh" [see Targum Onkelos on Ex. 30:23].) Thus, "[Mordecai] shouted a great shout" (Esther 4:1) for "the voice is good for [the quality of] spices". (Jerusalem Talmud, Yoma 23a)
And the taste was bitter to Haman, for [it was him, as] the Serpent, who seduced Eve [to sin essentially with] the sense of taste [and received his punishment from the reincarnation of Eve, measure for measure].
[From Benei Yisaschar, Adar 7:10]