I think you're being pedantic. The notion of mercenary carries a term of professional soldier to it - not just someone who happens to be compensated for risking their life in some capacity, but someone who willingly chooses it as a trade.
We're talking about fiction tropes, after all.
You mean, somebody like, say, a Commander Shepard?
If we're talking history, the point I'm making is that the dividing line between 'mercenary' and 'regular professional soldier' is so blurry as to be virtually useless (if it exists at all). If the definition is 'people that fight for pay' it encompasses nearly every fighter ever. If it is 'people who consciously choose to fight because of pay', that still encompasses an awful lot of people in the official state-sanctioned militaries. The Latino kid from southern California who enlists in the US Army to get a steady income, medical benefits, and maybe a college degree would be just as mercenary as the most traitorous condottiere by such a definition.
I think that a good dividing line, for academics at least, is that mercenaries have coherent and organized recruiting and command structures independent of the state-sanctioned ones for which they operate, and that they exist to conduct combat and/or security operations. This definition is more useful in time periods in which the state (and its military, and its monopoly on the use of violence) is relatively clearly defined, such as the modern era. It is not very useful in earlier eras. In early modern Europe, for example, almost every state relied entirely on hiring contractors to recruit units for service under a king (the origin of the terms 'colonel' and 'regiment'); suggesting that this meant that they were all mercenaries is kind of like saying that the state didn't have any revenue because it relied on tax farming instead of direct collection. It's an anachronistic distinction in such a context, one better employed to rhetorical effect than making an actual point. It also leads to some bizarre but amusing conclusions, such as the notion that the entire Prussian state military acted as mercenaries under the pay of the British in 1793-95 when fighting against revolutionary France, because the Prussian army in western Europe was explicitly supplied and paid at British behest and was therefore available for British orders, yet possessed a command structure independent of that of the British military. Given the moral impression most people have about mercenaries, that does not cast a particularly positive light on Prussian policy during that War of the First Coalition.
But you brought up tropes. In a literary sense, 'mercenary' essentially means what Jacob Taylor says about Thane in
Mass Effect 2: a person loyal to nothing but the next paycheck. It goes back to the image of the traitorous condottiere. It's a fundamentally pejorative label, a brush to tar somebody as immoral, irresponsible, and ignorant of any higher cause. It's difficult for me to imagine a fruitful conversation about whether somebody like that could plausibly be a knight in shining armor to anybody.
Now, obviously, there's an excellent reason why that label exists. It provided the state with ample justification for fully nationalizing all military and military-related assets. If everybody outside the state-sanctioned army was labeled to be a traitorous scumbag with no loyalty to anything but the next paycheck, there would be an outcry to make sure everybody was part of the state-sanctioned military. And while there are more than a few high-profile cases of 'mercenaries' deserting their masters for the highest bidder, that's happened plenty of times with state-sanctioned troops, too - individual desertion constantly, mass desertion on a number of highly embarrassing instances, even actively committing treason en masse like Vlasov's Russian Liberation Army (formed from Hiwis and Soviet PoWs) or the St. Patrick's Battalion of Irish-American deserters and traitors in the Mexican-American War.
It also ignores that many of these individual units did possess a great deal of spirit, elan, and devotion to a higher cause. Often in the Thirty Years' War, that higher cause was quite literal; many regiments were organized on religious lines and participated in communal religious activities like regular prayer, singing hymns on the march, playing
Te Deums after battles, and devoting glories gained in victories to the Christian God that they believed favored them. But even independent of that, the sorts of men who could fight alongside a comrade one day and then turn on him the next due to cashflow problems were exceptions, not the rule. Swiss mercenaries gained a great deal of reputation (and more than a little trouble) in the late medieval era for being almost uncompromisingly rigid about contract terms and regular pay, and would turn on a deadbeat king (and they were all deadbeats) if his enemies offered more money, but they were also seen as extremely special because of that.
If you're intent on the tropetastic version of mercenary, then I would have to say no, I can't envision a knight in shining armor being somebody with no cause beyond money. If you're intent on a more academic one, then I would have to say yes, they are absolutely valid KISAs, because the means of one's recruitment and supply doesn't have much to do with being a morally upright warrior. The question is extremely sensitive to the definition of mercenary you want to use, which is why I asked -
not as a sort of 'gotcha'.
A key part of the definition of mercenary is that they are foreign of the nation/group/etc that hires them.
That's not a universally employed definition. For example, most modern private military companies, or PMCs, are described as 'mercenaries' in modern press. Take the infamous American company Blackwater as an example. The Americans and Iraqis made use of Blackwater contractors in attempting to suppress insurgencies, during the course of which Blackwater security specialists were responsible for the Nisour Square massacre in 2007. The Blackwater employees who participated in that war crime were American citizens and tried under American law; they were not foreign, and since many of them at some point had served in the 'official' American military they weren't really outsiders by other definitions either. And judging by the actions of the regular American military during that conflict, it's not like you could blame the atrocity on the Blackwater operators' mercenary status, either.
This problem with definitions goes back to the earliest uses of the word 'mercenary'. The classical Greeks' word for 'mercenaries' was
misthophoroi, which literally means 'pay-bearer', or somebody who fights for pay. But originally, the term was clearly meant to emphasize men who fought outside the 'usual'
polis military system; it's nearly interchangeable with
epikouroi, or 'fighters alongside' (i.e. 'alongside the official
hoplitai'). This was an issue, because
poleis paid their normal fighters, too. Athenian oarmen, who were mythologized as the avatars of the spirit of democracy in opposition to the (supposed) slave armies of the Iranian shahanshah, happened to get at least three
obols a day;
hoplitai on campaign drew one
drachma daily.
Misthophoroi, going by many of those classical authors, were the root of everything wrong with Greece; their lust for silver extended every war, and at wars' conclusions they were knocked loose from every army and, with no other trade but fighting, they took the money of any despot who wanted them, which started all new wars - rinse and repeat. Again, it was a fundamentally pejorative distinction that rarely took reality into account, and certainly didn't admit that basically all soldiers got paid. In his
Philippic orations, the Athenian Demosthenes argued that his personal enemy, the king of Makedonia, Philippos, was a skillful general with a hardened army that possessed considerable advantages: it was fully professional, so it didn't need to return home for the harvest and could theoretically campaign through the winter given sufficient supplies. Demosthenes claimed that this was because Philippos paid his troops, in (supposedly) sharp contrast to Athens; he was effectively saying that the Makedonians did not fight fair. Philippos' army - a national Makedonian army, comprised of Philippos' subjects - was described as
misthophoroi for this purpose.
Or take the Swedish national army in the Thirty Years' War, recruited from Gustav Adolf's kingdom and shipped across the sea to Germany to fight in one of the most infamous quagmires in military history. After the luster of initial victories faded and after Gustav Adolf was killed in battle, the Swedish army actually held its own government hostage in what's often called the "Powder Barrel Mutiny" to force Queen Kristina and Chancellor Axel Oxenstierna to pay them properly and ensure that 'the contentment of the troops' was added to the peace terms when the end of the war finally came. It is hard to imagine a more stereotypically mercenary action. Yet most of those men came from Sweden. They were not outsiders to their country or society.
The dictionary definition really doesn't do much for us here.
There is another term that works better for what you're suggesting: foreign auxiliaries. Simply put, the Romans used the word
auxilia to refer to units that were recruited from non-citizen populations, and the term kind of stuck. Foreign auxiliaries can include private military companies, like the Catalan Company that fought for (and later against) the Byzantine Empire in the fourteenth century or the South African company Executive Outcomes that fought in Angola against UNITA in the 1990s, and they can also include forces that were part of the formal state-sanctioned military that happened to be from foreign places, like the Romans' units of
Germani or the Gurkha regiments in the British Army or the French
légion étrangère.
Combatants who work on a contract basis?
(Edit) actually support staff should go in that as well but I'm not sure how to word it.
Yeah, "combat contractors" isn't a bad definition, but I think it's unnecessarily broad in that it implies, but does not outright state, that those contractors should be (or possess) actual organizations.