Medieval knights are often portrayed as honourable and gushing with valour. Obviously it’s not wholly true for all, but is the concept that all warriors of kingdoms are fair and just, based off any specific event or person? Thank you.
This is a question with an incredibly vast scope, but I'll do my best to answer it in the context of early medieval Ireland.
Firstly, it's important to recognize the social function that an honour-system serves in a time and place like early Ireland; this was a rural, agrarian society that lacked strong, centralized states that could enforce laws and impose punishments on criminals. Yet, at the same time, Ireland had a sophisticated legal system that regulated everything from economic transactions between individuals, the care of mentally ill dependents, assault and even mocking another person.
Irish kings during this period (roughly the 8th to the 13th centuries) were essentially little more than military leaders who were more concerned with waging ceaseless war against one another than regulating trade and settling disputes (though there are several examples of kings becoming involved in the execution of the law during this period). So how does a complex legal system function when there is no real state apparatus to enforce it? This is where honour comes in.
From a sort of anthropological perspective, honour systems are most commonly found in societies and communities that either lack organized channels with which to settle disputes (states, courts, police etc.), or exist beyond those apparatuses (organized crime, for instance). In such contexts, honour is the mediating force that allows individuals to engage in social and economic interactions with eachother in the absence of coercive structures that would otherwise hold them accountable. In other words, in a stateless society that is simultaneously hierarchical and bound by rigid guidelines, honour is what lets people play nice with each other through trust. And because honour is so socially important in this role, one's honour must be defended at all costs; any slight against your honour could mean that you might lose the ability to form social and economic contracts, or even lose your social status.
Because honour had such an important social, legal and economic purpose in early Irish history, it was actually quantifiable according to one's social status. Every free individual was ascribed what is called an "honour-price" (log n-enech - lit. 'price of his face) that determined what compensation they were owed if they had been victim of a crime, what sort of food they were portioned at feasts and even how much weight their testimony carried in court. Unlike Roman law, the indigenous Irish legal system absolutely did not consider all "citizens" equal before the law - a king's testimony could overrule the testimony of a poor freeman. The principle function of the honour-price was to recompense a person for any damages done to their person, property or honour in the cases of murder, injury, rape, theft, refusal of hospitality and even satire, among the other examples mentioned above. Honour price was determined by one's social class, which was in turn determined by their wealth in cattle^^1 and the amount of clients/dependents they held beneath them. This ranged from a year-old heifer in the case of the poorest grade of freeman to 14 cumals (lit. a female slave, but is more often than not used in Irish legal texts as a denomination of cattle, in this case, 14 cumals is the equivalent of 42 milk-cows).
This system of honour prices ensured a sort of caste system, though upward and downward social mobility were possible. This is because an individual could not form contracts that exceeded their honour-price; ensuring that those lowest in the social hierarchy remained dependent on those at the top to maintain their cattle herds, while ensuring that the wealthy and powerful had a constant pool of manpower and labour at their disposal. Dependents of freemen did not possess their own honour price, but were entitled to usually half of their patriarch's.
I mentioned before that maintaining one's honour was crucially important in this society, and that is because even those at the height of the Irish social hierarchy were liable to lose their status is they were seen to forfeit their honour. Kings and lesser aristocrats, for example, could be dethroned or even be reduced to the status of a commoner if they showed cowardice in battle, were seen to give false or unfair judgments or even if they lied. Irish kingship maintained significant vestiges of the Indo-European tradition of sacral kingship into the historical period. This means that the king's role was not to be a powerful ruler, but rather to maintain the prosperity and metaphysical wellbeing of his kingdom and subjects through the maintenance of justice and abiding by rituals of kingly comportment (every king had ritual geassa, or taboos, that they were expected to perform or else they faced death or dethronement), explaining why lying and unjust kings would theoretically lose their honour-price. Even for commoners, maintaining one's honour through force was crucially important; Irish law allowed for the waging of blood feuds in the case that a person had been murdered and the victim's honour price had not been paid. Their kin group and clients were obliged to exact vengeance on the culprit and their family.
An interesting aside is the important role of the poet and satirist in early Irish society, due almost entire to the driving role of honour as a form of social cohesion; the former could enhance one's honour, while the latter could shatter it. Poetry and satire seem to have been regarded as something nearly akin to sorcery, and as late as the 15th century, Irish annalists ascribed the death of an English magistrate to a blistering poem composed against him by an Irish poet. Though it may be literary embellishment, there are numerous mentions to satire causing blemishes on the faces of those who had been targeted by a satirist. Even poets were stratified into different grades with different rights and duties, and the highest grade, the ollam, possessed an honour-price equivalent to a king; because of the Irish preoccupation with honour, poets and satirists were classed among the nemed, a caste of professionals like lawyers, judges and priests and were given special privileges such as the ability to safely travel between hostile kingdoms. Satire was taken so seriously that unwarranted satire was punishable by law, with the offender having to pay the victim their full honour-price; the Bretha Crolige, a Middle Irish law book, demonstrates that women who practiced "illegal" satire were especially revilved, grouping them among female werewolves and vagrants.
So, in sum, early medieval Ireland was a stateless society that was paradoxically incredibly hierarchical and bound by rigid social, economic and legal rules. In the absence of a state, an honour-system filled in as the social glue that allowed society to function.