Who is the blonde actress at end of avengers movie in celebration Montage?

You're likely thinking of Natalie Portman in the end-credit scene of *Avengers: Endgame*.

She appears as Jane Foster, Thor's love interest from the first *Thor* film. While not part of the main celebration montage, she appears at the very end, receiving Mjolnir from Thor, confirming her transformation into the Mighty Thor for the upcoming *Thor: Love and Thunder*.