Dena is the ying to Samantha's sometimes-evil yang.
She's the main character's neighbor and oldest friend. She was ignoring by an emerging Bad Sam in seventh grade... but the amnesia offers Dena a great opportunity to reclaim her old pal, causing Dena to be delighted to be welcomed back into Samantha's life.
Andrea: Listen to me, listen to me. You are ready for this. You look fabulous, the pond is deep, the fish are biting!
Dena (suddenly appearing): Well, hello!
Andrea: But then a boat motors through...
Samantha: Dena I'm not gonna stay somewhere when I'm not wanted.
Dena: It's not that hard, you get used to it.