Ever since 1.5.10 (as far as I'm aware, might have been added earlier) TaleWorlds silently added a way to stop clan members (that includes your companions as well as family members) from dying in combat.
How to activate it:
PPS: Thank you TaleWorlds
How to activate it:
- Open Campaign Options (Esc -> Campaign Options while in-game)
- Scroll down to "Clan Member Death Possibility in Battles" and set it to "Reduced by 100%"
- Enjoy!
PPS: Thank you TaleWorlds