What is the name of the unicorn who becomes an alicorn princess in 'Friendship Is Magic'?

The unicorn who becomes an alicorn princess in “Friendship Is Magic” is Twilight Sparkle.

Twilight begins the series as a highly gifted unicorn and Princess Celestia’s student. Celestia sends her to Ponyville, where Twilight learns about friendship alongside Applejack, Fluttershy, Pinkie Pie, Rainbow Dash, and Rarity. Together, they become known as the Mane Six, with Twilight representing the Element of Magic.

Her transformation happens in “Magical Mystery Cure,” the third-season finale, after Twilight repairs a spell by Starswirl the Bearded. The experience demonstrates that she has created new magic through friendship. Celestia then transforms her into an alicorn—a pony with both a horn and wings—and crowns her a princess.

A common mix-up is confusing Twilight with other alicorns such as Princess Celestia, Princess Luna, or Princess Cadance. Twilight is the one who starts as a unicorn and later becomes the Princess of Friendship. In the series finale, she eventually becomes ruler of Equestria.