I don't know if these are really appropriate, but there are a couple of things on the stunt and Power lists that you might find useful.
There's the Protector stunt for Athletics
Protector: You are an expert at the protection of others. Add two to your Athletics skill when using it to create a block to protect another character. (A similar stunt could exist in many other skills.)
and the Defender stunt for Weapons
Defender: You watch over not only yourself but others. Allies in your zone may take a supplemental action to hide behind you. If you take a supplemental action to protect your allies, they may use your Weapons skill to defend against melee attacks that exchange. You may take a single supplemental action each exchange to protect up to two allies. Both you and your allies may take these supplemental actions in response to an incoming attack, as long as you have not yet acted during that exchange.
and the Protective Instincts upgrade for
the homebrew version of Pack Instincts.
So yeah, those exist.
I came up with a stunt like that for a character of mine.
Taking The Hit - You're either trained as a bodyguard, or are otherwise just that selfless. Whenever an ally fails a defense and takes a hit that would require a Moderate or higher consequence to avoid being taken out, you may sacrifice your next turn to interpose yourself, taking the blow and rolling your own defense instead, taking the damage instead of your ally.
I never got any use out of it, though, so I can't really say if it's good or balanced or anything.
Seems kind of weak. Why limit it only to attacks that would require a Moderate Consequence to survive?
The fact that you never got to use it indicates that it's probably too narrow...assuming the game you made this for lasted for more than like one session.
If I were you, I'd go with something like...
Taking The Hit: You're either trained as a bodyguard, or are otherwise just that selfless. Whenever another character in your zone is targeted with a physical attack, you may sacrifice your next action to make yourself the target of that attack in their place. (This may lead to the attack targeting you twice, if it's a zone attack.)