What does 'ally' mean, as a game term?

Question

Most of the time, it should be really easy to determine who is my ally and who is not. The people who try to kill me are most likely my foes, whereas the people who entered the dungeon with me are probably my allies.

But sometimes it’s not that simple. What if one of my party members got replaced by an evil doppelganger, who plans to kill me in the near future? What if one of my allies infiltrated the court of the evil king and I accidentally have to fight her, while she is (unbeknownst to me) under the effect of a disguise self spell? What if a character fell in love with the BBEG and just wants to save him from himself, while the BBEG hates the character and actively tries to murder him?

Most spells and abilities that affect others specify the targets as “creatures” or “willing creatures”. Some explicitly affect allies only. That’s why I want to know, how to determine who’s allied with whom. Do I have to think that someone is my ally for someone to count as my ally? Does someone else have to believe to be my ally in order to count? Do both of us have to agree on being allies? Is there an objective observer who determines who’s allied? Can Person A be the ally of Person B, while Person B is not the ally of Person A?

There are many examples for spells/abilities that care about allies, among others:

• the Exalted Champion feature from the Oath of the Crown Paladin (SCAG, p. 132-133), that grants allies within 30 feet advantage on Wisdom saving throws.
• the Inspiring Surge feature from the Purple Dragon Knight Fighter (SCAG, p. 128), that allows one or two allies within 60 feet to make a melee weapon attack as a reaction.

With features like this, you could accidentally affect someone who’s not actually your ally. You could fail to affect someone who is actually your ally, but you don’t know it (yet). Or you could affect someone who doesn’t want to be affected (because they don’t want to be allied with you). This all depends on the interpretation of the word “ally”. I’m not sure if "ally" is a defined game term (but I don’t think so), or if I have to rely on the dictionary definition. That’s why I want to ask:

How do I determine who’s allied with whom?

Answer 1

Ally is not a precisely-defined term in 5e D&D.

You are unfortunately dependent on the normal English meaning of the word, and thus it is largely the DM's ruling as to whether or not two creatures count as allies or not on a case by case basis. The Oxford English Dictionary defines an ally (in this sense of the word) as:

A person or organization that cooperates with or helps another in a particular activity.

Personally, I would tend to rule that creatures are allies if they are co-operating together and would both agree that they are allies at that moment, and so abilities that affect allies do not affect creatures that you do not currently regard as allies or that do not regard you as an ally.

This does allow for alliances of convenience - two creatures who are entirely unfamiliar to each other but nonetheless find their lots thrown in together could treat each other as allies if they like. Say, for instance, two prisoners who do not know each other but are thrown into the arena together to face a beast - if they're willing to work together, they can be allies.

It also allows for deceptive alliances; say in the example of the doppelganger who has infiltrated the party. They may ultimately plan to betray the other members, but it's perfectly acceptable for them to consider them as allies and thus get the benefit of allied abilities up until the moment they turn upon them.

Making it simple

If the precise classification of who is and isn't an ally ends up taking up too much table time or prompts arguments, then as a house rule you could simplify such targeting rules by replacing references to "ally" with "willing creature you choose". There are a probably a few edge cases where this might cause a notable change in effect resolution, but it seems to me that in most circumstances it would result in targeting which was consistent with the intention of the ability.

Answer 2

An Ally is:

a friendly creature willing to risk themselves for your mutual goal

As Carcer states, the term "ally" is not explicitly defined within the rules. However, that does not mean we have to immediately jump to the normal English meaning of the word when there are other options of higher relevance¹. We can instead search for use of the word within the core rules and see if any context patterns emerge. In this case, there are so many uses of the word in other abilities that we can say with reasonable confidence what it means.

Being an ally is closely linked to being friendly

In contrast to being an "ally", being "friendly" is explicitly defined within the rules. The PHB tells us what being friendly means (p. 185):

In general terms, an NPC’s attitude toward you is described as friendly, indifferent, or hostile. Friendly NPCs are predisposed to help you, and hostile ones are inclined to get in your way

And the DMG adds to this understanding (p. 244):

A friendly creature wants to help the adventurers and wishes for them to succeed. For tasks or actions that require no particular risk, effort, or cost, friendly creatures usually help without question. If an element of personal risk is involved, a successful Charisma check might be required to convince a friendly creature to take that risk.

So by definition, a friendly creature wants you to succeed, but is not always willing to risk itself to help you. Since we have a good definition of what 'friendly' means, we can now note that about a third of the time when the PHB mentions allies, it also mentions them as being friendly. Clearly the two terms are related:

PHB, Help Action, emphases mine:

Alternatively, you can aid a friendly creature in attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally's attack more effective. If your ally attacks the target before your next turn, the first attack roll is made with advantage.

PHB, Forest Gnome (p. 37), emphases mine:

Forest gnomes tend to be friendly with other good-spirited woodland folk, and they regard elves and good fey as their most important allies. These gnomes also befriend small forest animals and rely on them for information about threats that might prowl their lands.

Similar links between 'friendly' and 'allied' can be found in the descriptions of Character Building (Building Bruenor, Step 4), the Tiefling Race details, and the Bard Class Feature, Song of Rest

Thus it is clear that being an ally is linked to being friendly.

However, there are other places in the PHB where being an ally is mentioned without an explicit mention of friendliness. We can look to these to see whether it is ever possible to be an ally without being friendly. A review of who a Bard inspires (p. 54, in two places), the functions of the Cleric class (p.56), who can benefit from a Battlemaster's Distracting Strike (p.84), who a Ranger can teach (p.90), who can benefit from a Wizard's Illusory Realty (p. 118), who a flawed Folk Hero has trouble trusting (p.132), who might be healed with the Healer feat (p. 167), and who can be targeted with an Aid spell (p.211) finds that in no case is it suggested that an ally would not be friendly.

It appears that allies are always friendly. If so, what then is the difference between an ally and a friend? Here we can return to the DMG definition of friendly, and note that while a friendly creature always wants its friends to succeed, it is not always willing to risk itself to help. So one possible definition of ally would be that while a friendly creature wants you to succeed, an ally is a friendly creature that is furthermore willing to incur risk, effort, or cost to itself in order to help you succeed.

This proposed definition, that an ally is willing to incur personal risk on your behalf, is consistent with all the uses of 'ally' in the PHB. It is especially congruent with the definition of the Help action, where you must place yourself within five feet of a foe to give your ally advantage. It makes sense with the feature Pack Tactics (found in the PHB descriptions of Dire Wolves, Lions, Reef Sharks, and Wolves), which confers advantage on your attacks when an ally of yours is willing to risk themselves by being adjacent to your shared foe. And it even touches on the distinction made in the description of Forest Gnomes, where small woodland creatures are just friends, but more combat-ready fey and elves are not only friends but allies as well.

Being an ally means being willing to incur risk on someone's behalf

If we accept this as a definition, then ruling on whether a particular creature counts as an ally for a particular feature comes down to their willingness to accept risk on your behalf (or vice versa).

In the complicated circumstances suggested by the original question (a doppelganger, a spy, an unrequited love) the DM can rule about the sincerity of their feeling, or the PC's feeling, even if it is misplaced or mistaken.

Exalted Champion
The source of a Paladin's power is their Oath - their deep personal commitment to their values. Like their spells, the Exalted Champion ability is a function of the paladin's belief.

Your allies have advantage on death saving throws while within 30 feet of you. You have advantage on Wisdom saving throws, as do your allies within 30 feet of you.

Who does the paladin consider their ally? Who are they willing to risk themselves for? It might be a mistake for the paladin to risk themselves on behalf of the doppelganger or the BBEG, but if they are willing to do so, and if doing so advances their mutual goal, then those individuals are indeed allies, at least within the context of a specific encounter. Likewise, in their fight against a spy who is an unrecognized compatriot, the paladin would not be willing to risk themselves on behalf of someone they believe to be their foe. The spy would gain no advantage from exalted champion so long as the paladin did not believe it should extend it to them².

Inspiring Surge
Purple Dragon Knights (or, outside the Forgotten Realms, bannerets) are not paladins. Rather, they are Fighter class characters, often with high charismas. A knight "inspires greatness in others by committing brave deeds in battle." Thus, while a paladin's abilities are powered by their own Oath, a Purple Dragon Knight's abilities arise from the reactions of others that see them. As such, the belief of the observer is as important as the belief of the Knight. In order for a creature to benefit from a Knight's ability, it must be sincerely inspired. Thus, a DM could rule that a doppelganger or a BBEG would not benefit from the Knight's ability, while their unrecognized friend would. This distinction can even be seen in the specific wording of the ability:

Starting at 10th level, when you use your Action Surge feature, you can choose one creature within 60 feet of you that is allied with you. That creature can make one melee or ranged weapon attack with its reaction, provided that it can see or hear you.

Notice that the Exalted Champion benefits the allies of the paladin, those whom the paladin considers allies (regardless of how they feel about the paladin). The Inspired Surge, however, benefits those that are allied with the Knight, that is, those who consider themselves allies of the Knight. The Knight still must choose the creature, but unless the Knight's feelings are reciprocated, the ability as written shouldn't work.

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¹_{Personally, I try to work within the following hierarchical framework when interpreting a word: (1) Is it a game term? If not, (2) Are there multiple, consistent context clues from its use in core books or official rulings? If not, (3) can designer intent be inferred through unofficial statements? If not, (4) what normal English meaning seems closest to the way it is used within the rules?}

²_{As a divine caster, there is also fertile ground for a DM to subvert this ruling if the paladin has a god as well as an Oath. If the doppelganger does not get advantage on their Wisdom save but the disguised spy does, the paladin would do well to ask themselves, 'what is my God trying to tell me here?'}