The thing, Stiles is totally head over heels for Derek. He just knows (thinks) Derek just wants to be friends. And, you know, maybe uses it as an excuse to hopefully make Derek jealous in hopes Derek might realize how absolutely perfect Stiles is for him.
So Stiles does what he does best. He makes shit up. "Her name's Elena. I met her through her brother, who I met a few months ago. I hang out with him a lot and we go between hanging out here and down where they live. She's... really great," he says, smiling a little at the memory.
"A while back, my dad was in the hospital. It was pretty horrible and we were all scared, and he's okay now, totally. Just has to watch what he eats from now on, you know? Mom's gone nuts with the health food shtick. But she was there for me, came up with my favorite snacks and we just vegged out on the sofa and watched movies all night."