Knight

In came Sir Seabhac, a knight of great renown. She knew him well. Leader of Marquis Flameheart's personal guard, he had arrived at the negotiation table on his master's behalf multiple times during the war. Baroness Aoife had served as the Marquis's sword, but Sir Seabhac had been his shield.

— Chapter 16.1 in Iceburn

All throughout the kingdom of Mǣdweburh, as well as many other countries, there are plenty of people longing to become knights. Even professional soldiers which are paid to be on the ready at all times, still would like the promotion. Some want to be a knight for the money and glory, others for the possibility to fully serve their liege. And there is the advantage that when you serve a noble as a knight, they may teach you secret mana techniques.

People

Noble non-heir children
Talented soldiers
Recognised individuals

Ability

In Mǣdweburh, always magically talented
Elsewhere, varied, depending on country

Rank

Considered untitled nobility

Recruitment

Needless to say, how knights are recruited differs between countries and even between parts of the same country. Throughout the kingdom of Mǣdweburh, the first condition is magical talent. A soldier or non-heir noble can forget about becoming a knight if they cannot properly wield mana. At the very least, the ability to sense enemy mana usage is a must.

If recognised, normally one becomes a prospect knight, or a squire if discovered at a young age. Training then is meant to verify the person's skills. Once considered skilled and talented enough, they will be knighted and given the opportunity to pledge their loyalty to their liege. At that point, they no longer need to take care of their own needs, such as food or gear. All of that now falls on their liege.

Given how many more advanced mana techniques are kept classified, the opportunity to learn them is a great motivator to knights. Being taught these techniques shows they hold the trust of their liege, which many a knight will be extremely grateful for. Even for the children of nobles, being recognised is a great honour.

Responsibilities

Lower nobles their knights tend to be simply elite soldiers. But the higher ranked the noble is, the greater the burdens are that their knights are expected to carry. They may need to lead troops into battle, or stand by their liege, ready to hold off an assassin's blade. Some nobles have personal guards, which are a step up. These will venture forth into combat by the side of their liege, risking their life in blind loyalty.

A high noble's personal knights also may need to learn etiquette and represent their liege in many a situation. They might need to negotiate on their liege's behalf, advise them in both war and peace, train their heirs, or attend court. A trusted elite knight can act as their liege's sword or shield, though not many nobles are willing to trust anyone that much.

Incidentally, knights also are a status symbol to nobles, not just when it comes to their skills, but also their number and gear quality. A Baron might be lucky to be able to maintain a single dozen of knights, whereas a Duke might pay upkeep on close to a hundred knights that answer solely to them. Private armies are crucial in hostile times, after all.