He-Man and She-Ra:
The Secret of the Sword
(1985)

Adora is actually Adam (better known as He-Man)'s twin sister who was stolen by Hordak as a baby, and she uses the Sword of Protection to transform into She-Ra. Using her strength, she manages to rescue He-Man and stop Hordak from firing the Magnabeam (her horse Spirit also transforms into Swift Wind, a flying unicorn).

The two return to Eternia (thanks to the Sorceress' magic) so Adora can reunite with her parents, however Hordak follows and teams up with Skeletor to capture Adora. Although they succeed, Skeletor betrays Hordak by sending him back to Eternia however He-Man quickly rescues Adora and she transforms into She-Ra to escape. Adora decides to return to Etheria to lead the Great Rebellion against the Horde, and Adam follows her to help out for a while. As He-Man and She-Ra, the two manage to score a huge victory for the rebellion.

In the end, He-Man and She-Ra tearfully say goodbye as he returns to Eternia.

Thanks Mark I!