What is the interaction between green-flame blade and mirror image?

Question

We are planning an attack on our nemesis. Intelligence efforts indicate that the enemy can use mirror image. I have green-flame blade. As we thought through our tactics the question came up:
Would GFB, on a hit, remove more than one of the mirror images? We think not, but we think that it will by default do damage to mirror image's caster.

What we hope happens: If I hit the image, the additional green flame hits the caster

[...] make a melee attack with it against one creature within 5 feet of you. On a hit, the target suffers the weapon attack's normal effects, and you can cause green fire to leap from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting ability modifier.

The only creature within five feet (assuming that I am one-on-one with our nemesis) is the one who cast mirror image. If I hit them, we think that the green flame does not spread to one of the images (nor damage them) but if I end up hitting one of the three images, the only creature available for the flame to hit is the caster himself. So it does so, by default. mirror image states:

Three illusory duplicates of yourself appear in your space. Until the spell ends, the duplicates move with you and mimic your actions, shifting position so it's impossible to track which image is real. You can use your action to dismiss the illusory duplicates.

Each time a creature targets you with an attack during the spell's duration, roll a d20 to determine whether the attack instead targets one of your duplicates.

The additional flame damage, we are pretty sure, won't hurt any of the duplicates.

If an attack hits a duplicate, the duplicate is destroyed. A duplicate can be destroyed only by an attack that hits it. It ignores all other damage and effects. The spell ends when all three duplicates are destroyed.

Example of how we think it works: sixth level caster, cha of 18, attacks with GFB, hits. Duplicate destroyed, 1d8+4 green flame damages adjacent creature (the caster of mirror image).

Question restated

If my GFB hits a mirror image duplicate with the attack, does the additional damage automatically damage the creature who cast mirror image? (As long as no ally of that creature is within 5')
or
Does it maybe hit (to no effect) one of the other duplicates?
(The related consideration is how does one discern what it damages: the caster or the duplicate?)

Related question

Answer 1

A mirror image duplicate is not a creature, so the condition of the additional damage is not satisfied.

Green-flame blade states:

make a melee attack with it against one creature within 5 feet of you. On a hit, the target suffers the weapon attack’s normal effects, and you can cause green fire to leap from the target to a different creature of your choice that you can see within 5 feet of it.

The condition that must be satisfied for the green fire to leap is “a hit with a melee attack against one creature”. When the description says “on a hit”, it is obviously referring to a hit against the creature mentioned in the previous sentence. Since the mirror image duplicate is not a creature, the green flame does not leap.

Answer 2

Whether the additional damage triggers at all is debatable.

Green-flame blade requires you to make an attack against a creature. You can do that if the creature is protected by mirror image; the attack might end up hitting one of the duplicates but it's aimed at the caster.

But GFB's additional splash damage only happens "on a hit". Since the attack was required to be aimed at a creature, one interpretation of the spell is that it only works if it in fact hits a creature (rather than an illusion of a creature).

Another reading of it would be that you must attack a creature, but the attack just needs to hit something.

I don't see any way with the language of the spell to choose one of those interpretations over the other. Since this interaction is pretty specific to the case where a non-creature might take a hit for a creature (which is mirror image and... edge cases of the "hitting cover" optional rule) there aren't really game balance concerns about it, either.

This won't guarantee that the splash damage hits the caster.

The point of the mirror image spell is that the images are indistinguishable. They are described as "duplicates" and the spell says that "it's impossible to track which image is real". It further explains that it doesn't affect creatures that can't see or that rely on a sense other than sight, which implies that, if you are relying on sight, the images will confuse you.

So when a spell like green-flame blade tells you to choose a creature you can see, you don't get to say "that guy, and not one of his duplicates". From your perspective they are all creatures you can see, with an equal chance of being the real guy.

It won't destroy an additional duplicate.

The duplicates are specifically only affected by attacks, and this isn't an attack.

Answer 3

The Green Flame will destroy that duplicate and do nothing else.

If an attack hits a duplicate, the duplicate is destroyed. A duplicate can be destroyed only by an attack that hits it. It ignores all other damage and effects. The spell ends when all three duplicates are destroyed.

Assuming you target a duplicate (75% chance) the duplicate will be destroyed and any other effects or damage will be ignored.

Answer 4

Hitting a duplicate with green-flame blade can probably damage the caster of mirror image

The green-flame blade spell states:

[...] On a hit, the target suffers the weapon attack's normal effects, and you can cause green fire to leap from the target to a different creature of your choice that you can see within 5 feet of it. [...]

And reading mirror image we note the following:

[...] A duplicate's AC equals 10 + your Dexterity modifier. If an attack hits a duplicate, the duplicate is destroyed. A duplicate can be destroyed only by an attack that hits it. [...]

Thus, we do actually hit a duplicate with the attack made as part of green-flame blade and thus destroy that duplicate. We then can also choose a creature other than the target (the duplicate) to cause green fire to leap to. Because the duplicate is not actually the caster (they are separate entities entirely), we can choose the caster as the target for green-flame blade's bouncing flame.

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Honestly, this works because nothing says it doesn't work. No part of these rules lead to a contradiction or problematic thing, it all just works out as written. You hit the duplicate and cause fire to leap to a creature of your choice near it, and fortunately, the caster of mirror image is a nearby creature we can choose.

Thus, the duplicate is destroyed and the caster of mirror image takes damage from the bouncing green flame.

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One reason this might not work

As Thomas Markov brings up in their own answer, it is also possible that green-flame blade bouncing actually requires hitting a creature initially. This is because the text of the spell states (emphasis mine):

[...] You brandish the weapon used in the spell's casting and make a melee attack with it against one creature within 5 feet of you. On a hit, the target suffers the weapon attack's normal effects, and you can cause green fire to leap from the target to a different creature of your choice that you can see within 5 feet of it. [...]

The two bolded parts may actually result in a requirement that the spell must initially target a creature in order for the bouncing effect to occur at all. If this is the case, then hitting a duplicate with green-flame blade does not result in the caster of mirror image taking damage.

Personally, I would rule that the retargeting effect of mirror image overrules this entirely, the same way it allows the attack to target the non-creature duplicate in the first place. But a GM could easily rule otherwise and decide that the spell cannot target the duplicate at all, or that it can and thus destroys the duplicate but also doesn't cause flames to bounce to the caster of mirror image.