Which pony is the princess of love and rules the Crystal Empire?

The pony who is the princess of love and rules the Crystal Empire is Princess Cadance.

Princess Cadance, formally Mi Amore Cadenza, is an alicorn associated with love, compassion, and healing relationships. She first becomes prominent in the second-season finale, “A Canterlot Wedding,” where she marries Shining Armor, Twilight Sparkle’s older brother. The wedding is disrupted when Queen Chrysalis impersonates Cadance and attempts to exploit Shining Armor’s love.

Cadance’s connection to the Crystal Empire becomes central in the third-season premiere, “The Crystal Empire.” She and Shining Armor protect the empire from the dark unicorn King Sombra, with Cadance exhausting herself to maintain a magical barrier. Together with the Crystal Heart, her love helps repel Sombra’s corruption and restore hope to the Crystal Ponies.

Princess Luna and Princess Celestia are royal alicorns, but they are not the Crystal Empire’s ruler. Queen Chrysalis is Cadance’s enemy, not a princess or a benevolent ruler.