Diamond Lotus
While originally all sirens were sent into the mirror (Equestria girls world), during Orchid Rose's reign she was able to reform them and allow them back into the world of Equestria. With the help of the changelings and Onyx and Sapphire, they were able to convince the sirens of alternative food methods so that they would no longer need go feed on magical energy stolen from others.
Now being able to get their energy and power from positive methods, the sirens re-establish their community in the South Luna Ocean. Sirens then would spend the time following their return and reformation, making amends and alliances with other species - though the seaponies are still not fond of them.
Despite technically not having cutiemarks, the elements of harmony picked diamond to be the element of altruism, for her determination of making friends and proving herself to be kind in spite of her species' past. Being an aquatic creature, land navigation is only temporary for sirens. But a permanent water-bubble spell from Orchid allows Diamond to freely be out of water for as long as she wants.
