Sigune is the boss of Chapter 19A: Bitter Cold in Fire Emblem: The Binding Blade. She is the flightleader of the Pegasus Knights who have turned to Bern, and plans to use Ilia's harsh climate to ambush Roy and attack him.
She was Juno's close friend, and Juno laments Sigune's death. This makes her wish for a brighter future in which Ilia would find other economical system instead of producing mercenaries, a future in which they wouldn't need to face each other nor hear of their friends death.
Sigune's stats are extremely well rounded and her weapon is potent, but a combination of factors make her far too easy to defeat. Her impressive movement and flight are wasted on a stationary boss, and her flight prevents her from taking full advantage of the gate on which she is stationed. Like most of Ilia's Pegasus Knights and Falcon Knights, she rides into battle over-encumbered, her Spear slowing down her Attack Speed by a full three points. Most well-trained bow units will have no trouble plowing right through her, especially if they are armed with a Brave Bow or have enough speed to double her, though doubling her might be a difficult prospect on hard mode.
Sigune: You... You're Juno's youngest sister.
Sigune: My honor as a knight of Ilia is at stake... Don't expect me to hold back.
Shanna: I know...
Sigune: Thea? What are you doing here? Oh...you've joined Etruria.
Thea: Captain Sigune...
Sigune: You know the code of the Ilian knights. You will get no mercy from me.
—Sigune's death quote in The Binding Blade.
The name Sigune comes from Arthurian legend. In some versions, she is Percival's cousin.