I know it's not recommended to create a friendly NPC to interfere in combat incounters.
But...
I have a group of players who are completely new to table top RPGs (so am I) and one in particular who is very used to video games, and judging from his Rocky/Rambo inspired character, very likely to get in over his head and pick fights he can't win.
I was thinking of starting them off with a guide NPC that is slightly stronger than their starting PCs and sacrificing him to let them escape from an overwhelming enemy incounter. Demonstrating via actual dice rolls that life is fragile in this game, and running in guns blazing is going to get you killed.