That's Izzy's older sister, Alicia. Her presence so far has been more implied than anything else. She's Dominique and Lucas's mom. It's been a long time since we've seen them but I can guarantee you'll see them again soon. Anyway, Alicia is 5 years older than Izzy, so she's about 13 years old here.
Alicia has always found Izzy more than a little annoying. Izzy's fixation on superheroes and her tendency to talk a lot wears on Alicia a lot. There's been more than a few times where she would've done anything to take a break from Izzy. But this isn't how she wanted it to happen. The aftermath of the events of Izzy's 8th birthday left Alicia with a lot of conflicted feelings. She finally got the peace she desired but possibly at the cost of her sister's life.
At this point, Alicia would love for Izzy to carry on about superheroes to her heart's content. But Izzy is still a bit of a ways from that.
Of course eventually Alicia will go back to being annoyed by Izzy again. And for reasons more complex than Izzy simply being annoying. But for right now she's just happy Izzy's awake again. It got really close there.