Two words: Cooking Incident
When Nat was a little girl she was reading from a cookbook and wanted to make something out of it.
She puts a stool in front of the stove and climbs on it.
She turns on the stove and puts a pan on it.
Unfortunately she ends up loosing her balance on the stool and falls.
When she falls the pan ends up falling as well and lands on the side of her neck.
Afterwards she felt a great deal of pain and had to use several ice packs.
The injury healed but the mark didn't.
She ended up having a permanent burnt scar on her neck.
So she always covers her neck because she is very insecure about it and refuses to go near stoves or anything that has fire and will only use microwaves.