Nausicaa spent most of the pre-Marching phase of UnS on a long expedition to Leng. In the company of Keitaro, Kentaro, and Naru, Nausicaa investigated the tower of Taylor d'Arden and the tower of The Great Lo.
In Taylor's Tower, they found the Black Heart Prophecy and some information on his plans and misdeeds, but...well, details aren't clear, but the tower was destroyed, many of the books were destroyed, and everyone on the expedition tends to twitch at the words 'pie'.
At the Great Lo's Tower, they slew what was probably The Great Lo.
At Marching, she apparently swore fealty to Prince Lucien and vanished into shadow with him.
Lucien, probably under outside influence, turned on Nausicaa and had his female lieutenant Kitiara attack her, before leaving her behind to face the construct known as the Black Crown.
She is currently riding with her father Mikage and his retainer Falme into Chaos.
Nausicaa has resurfaced recently, having suffered hideous injuries in battle with House Formorian, when she and her father assisted in purging the house of Outsider cultists.
Joining the defense of the High Summer Rites against hordes of Wu-Chaosians, she was defeated by Wu-Lord Formorian, then nearly turned into a Wu by Wu-Anno. Saved from that, she is currently recuperating in a field hospital in Chaos. She has been accompanied by Kaede Galfordson as of late.