Okay, we need to find the title of a graphic novel, based on Youngest's childhood memories. Published prior to 2016ish, available in US libraries.
"Auburn/red haired female character, probably the main. A boy who lives in the woods who is friends with a bear. All three sleep in a cave together at some point. A lich monster, a stampede of horses towards the end. A grandmother figure." Any of these points could be off a little.
Youngest continues: She is not from the woods. She got lost and is not happy to be in the woods. This little freak of a kid offers to let her sleep in his cave. Not futuristic, not Wild West, there are swords and wear fur but might have indoor plumbing.