Yasha and Kaya's mana loss reduction makes Medusa's Mana Shield ability consume less mana, hence making her more durable. Its agility and attack speed bonus helps Medusa's damage output and improves her farming potential. However, this item should be considered very early on the game or not considered at all.
Yasha and Kaya's movement speed bonus helps Mirana roam, its agility and attack speed bonus increase her damage, and its spell mana loss reduction and spell amplification make Starstorm and Sacred Arrow more effective.