Summer Girl
Siobhan is one of the few Summer Girls named. When we first hear of her, Beira is taunting Keenan, saying she may be his Summer Queen, but had lacked the courage to pick up the staff and so never found out. It's not clear from this conversation how old Siobhan may be, however.
The Summer Girls, in general, keep close to Keenan and are loyal to him. Niall, one of Keenan's advisors, explains the changes a mortal girl undergoes when she becomes a summer girl, noting:
"Once they're Summer Girls, their inhibitions are gone." He also tells Aislinn the Summer Girls can and do say "no" to the guards and other members of the Court -- but not to Keenan. Even Donia notes the Summer Girls are like plants needing to be near Keenan to survive.
Like all the Summer Girls, Siobhan has what appears to be vine tatoos running over her arms and legs, but these seem alive, undulating over the girls' skin.
More description has been provided by Melissa: "Siobhan. When she's not adopting a glamour for various reasons . . . Dancer-thin, tall-ish, long straight mid-tone brown hair that looks like the ends need trimmed a little (ie not one of those perfectly trimmed in a straight line cuts). Her hair falls mid-to-low back. Brown eyes. Serious look when she thinks about . . . umm, someone. Kinda wistful. Hands like a pianist."
A Siobhan excerpt can be found in reply #2,
here.