Rosetta is Joshua's partner. She is known to be exceptionally talented in magic and can even use the highest level of magical kisses, The Princess Kiss.
(Source: Magico Wiki)
Rosetta and Joshua compete with Emma and Shion in the couple contest.
She was exceptionally talented at magic and was sold by her parents to the king of Dragon Tail for a large sum of money. The king is a 200-year-old monster, who has around 100 different wives. He has each wife learn "The Princess Kiss" so he can continue to replenish his life force.
Before she married, She fell in love with the leader of the Knight Order, Joshua. They eventually ran away together and eloped.