Miyuki and Yumeiko aren't immortal, they're just not afflicted by death due to age. They can still very much die though, from a number of reasons that include, but aren't limited to:

- Violence
- Starvation
- Suffocation
- Dehydration
- Blunt force trauma
- Decapitation
- Bloodloss
- Truck-kun
- Explosions
- Magic
- Exorcism (Though this likely only appplies to Miyuki and not Yumeiko, as the latter is a divine fox and not technically a 'yooukai')
- Impalement