Childrens Secret Valkyrie Society
I read this round maybe 2016-2023 from i think my library. Its a middle school or upper elementry level novel.
MC is about 12
Her mother died in a suspicious house fire
Her father trained her in martial arts
The basement contains lots of weapons
She is home alone training when she hears a noise upstairs and investigates
She quickly moves to another state with her father
They are pursued by some vehicle that attacks them on the road and crash, leaving Dad hospitalized
A nurse introduces her to a hidden Valkyrie/supernatural society by showing her the nurse's own tattoo—the MC doesn't have one
Nurse's son walks her to school
The strange traffic scene is not supernatural; it's simply a local custom where pedestrians have extreme right-of-way
She makes two friends at school
One is a girl who secretly has real feathered wings, concealed under/with a little-girl backpack
There is a secret elevator in the school
It descends into a large, friendly underground city/community beneath the town, with ordinary places such as cafés, parks, meeting rooms, etc.