She has a shaping power that she uses to place stars over her daughters and to briefly see the future. It's much rarer than the average shaping power because it's a mix of Ellowine enchanting and Necronium weaving (illusions placed within the sky).
Sixty years ago, she had dark brown hair that was put up in an elaborate style. In Book 3, Crystal Keepers, Cole sees her in Junction City, and notes how her hair has a few gray streaks, and her eyes were tired looking. Nevertheless, she still looked quite regal and queenly.
Book 2, Rogue Knight Edit
While Mira, Jace, and Cole were in Trillian the Torivor's palace, they had to go through simulations of their worst memories to find Honor and rescue her. Mira's memory was of the day her father faked her and her sisters' deaths and stole their powers, sixty years ago. In this memory illusion, Harmony was portrayed as an illusion of how she looked the last time her daughter saw her.
Book 3, Crystal Keepers Edit
When Cole is sent by the Unseen to talk to her in Junction City, Harmony finally got to meet him in person. After telling him the code words that implied his allegiance with his daughters and the rebellion, she was much less formal and worriedly asked about the fate of her daughters. She was able to provide him with vital information regarding the location and workings of Abram Trench, the Grand Shaper of Zeropolis. Since he had kidnapped Constance years before, and was the only Grand Shaper under Stafford's influence, she knew secrets about where her daughter was being kept. Before leaving, Harmony told Cole that she was grateful for his assistance to Mira and the rebellion.