If the player character murders an innocent, their affinity with their current companion will drop by 10% and will trigger dialogue with the player character expressing anger and warning them to never do it again.

If the Sole Survivor fires a weapon in a settlement, any companions present in that settlement will often become annoyed and make backbiting comments on how that is best saved for another time.

Piper, Cait, Preston, Macready, Old Longfellow and Porter Gage though being human are naturally immune to radiation but they will still take radiation damage from a gamma gun and radium rifle when used by enemies.

Some companions lose their essential status upon reaching the lowest level of affinity and leaving the player character.

They do, however, remain protected like other settlers and cannot be killed by NPC attacks (only by the player or their active companion or related action)

Some companions lose their essential status when dismissed.

If they're not healed within a set period of time, or when the player travels beyond a certain distance, they will leave and return to their last used home base.

If one is playing in Survival mode, the companion must be healed when incapacitated, even after combat.

If incapacitated, they will automatically heal after combat ends. Unlike Fallout 3, companions are marked as 'essential' when acting as active companions, meaning they cannot be killed.The game has a number of temporary companions that will only be available during certain quests. Steal Owned Items (may upgrade to Hack or Lockpick)

Fallout 4 valkyrie companion