Also known as the Nameless Ones, the Ysandir are an ancient race of immortals that have been confined to the Black City. There are ten of them, tall men and women, "so beautiful that it hurt to look at them for very long. Their power flashed and rippled around their bodies in a dance of green light" (7.101).
In other words, they should be setting off your freaky alarms.
Despite being called the Nameless Ones, they actually do have names. We meet three of them: Ylon, Ylira, and Ylanda, who are among the hungriest, and the first to make moves on Alanna and Jonathan. The Ysandir are cruel and powerful, and they don't hesitate to throw around their power to intimidate the two kids.
When Alanna lets her guard down for a second, letting Ylanda peek inside her head, Ylanda cracks up, saying, "In all my centuries…I have not known such a jest!" (7.122). The jest, of course, refers to Alanna's gender masquerade. (LOL.) Not that the Ysandirs' attempt at dividing the kids' loyalty or attention works: the kids remember the Ysandirs hate fire, so they summon up some magical blazes to defeat the immortals.
It's not easy, though. The two remaining Ysandir put up a fight: "Needle-sharp bolts of magic were lancing into their interlocked hands. Pain shot through their physical shells. Ylon and Ylanda stood before them in a wheel of yellow-green power. Fire streamed from them and broke on the newly formed globe of magic that held the bodies of Jon and Alanna" (7.171). Which leaves us with two conclusions: