Mermaid - physical differences
Voice: She generally shifts to a more melodious tone when beneath the water.
Mermaid's tail - Alysse's is red. It's generally more like a dolphin's than an actual fish, with the genitalia visible.
Skin: In mermaid form, Alysse's skin is slightly paler, and much more sensitive. Although useful for sensing ocean currents, it makes clothing uncomfortable - mermaids are used to having nothing but the water on their skin. (As a human, her skin is still more sensitive than a normal persons.)
Eyes: Her eyes are green in mermaid form, like her mother's were.
Breasts: A mermaids breasts are generally softer than a humans; they can double as air sacs, even inflating a little when required. This helps to counterbalance the weight of their tail (and generally makes mermaids better swimmers than their male counterparts.