This Symphonia belongs to Nesta Archeron.
A Symphonia is described as a silver ball the size of a chicken egg, round save for one flattened area so it might be set upon a surface and not roll. Tapping the top, makes music play. The device can trap music within itself, and play it back.
Cassian gifts Nesta Archeron this Symphonia for Winter Solstice. He gives her the present in a black velvet box while the talk privately in the River House after the gathering. Cassian explains how it works and tells her to tap the top of it, music starts playing, and Nesta recognizes the waltzes from the ball they had attended the other night, the recording clean of any of the crowd's noise.
Nesta asks him how he managed to get the crowd's noise out when he trapped the sound the night of the ball and he explains that he went back the day after the ball and asked the musicians of the Hewn City to play it all again for him, along with some of their favorite pieces. He then went to some of Nesta's favorite taverns to find the musicians and had them play music.
Nesta breaks down crying, touched by his thoughtful gesture, but tells him she cannot accept it, to which he tells her that it was made for her.
Later, Nesta brings her Symphonia with her to the training ring of the House of Wind each morning for some extra motivation while exercising, and also uses it when she has the House conjure six frogs which dance to waltzes provided by her Symphonia.
She plays it whenever she can.