Knights are a class exclusive to Lawful Good orthodox followers of Titanius, and while they are not fully exclusive to Humans or nobility, the majority of Knights do belong to those two groups, for several reasons.
The idea of Knighthood is fundamentally a Human thing. It originates from the specifically feudal structure of the Kingdom of Etrand, which itself in many ways originated as a compromise between the adminsitration model imported from Froturn and the pre-existing tribal ways. Froturn with it's professional mercenary legion just has a different structure - Knighthood is an alien concept to High Elves and non-Humans in general - but not the Half-Elves of Artaburro.
Normally, knights come from Etrandish, Etrancoasti or Artaburran nobility, but not always. Very often, the knighthood adopts orphan children and trains them to be knights - this is how non-noble Humans, especially Half-Elves (other than the ones from Artaburro), and also Half-Orcs, and even the various non-Human minorities of Etrand manage to make up a small percentage of the knights.
Knights who stray away from the Path of Light will usually become Death Knights.
Being a Knight comes with the bonuses of receiving all armour skills, an advanced skill at swordsmanship and blunt weapons, an advanced skill at polearms.
In the video game adaptations of the FRPG, Knights are a compromise between Warriors and Clerics, in the same way Battlemages are between Warriors and Magicians. They have higher amount of hitpoints than Inquisitors, they can use all non-ranged weapons, but have lower amount of spellpoints, they are also limited to chainmail armour at strongest.