Medieval European knights swore loyalty to their lords.
In the story, a young knight protected the princess.
mounted samurai; samurai on horseback
Historical use for a Japanese warrior on horseback; 騎馬武者 is often clearer in modern Japanese.
In history books, a samurai on horseback is sometimes called 騎士.
The king knighted him.
The knight swore an oath of loyalty to his lord.
騎馬武者 is the clearer modern term for a mounted Japanese warrior, while 騎士 in this historical sense is a more literary or older way to say the same thing.
武士 refers broadly to any Japanese warrior regardless of whether they fight on foot, whereas 騎士 in this sense specifically means one who fights on horseback.
騎士 (a knight) and 棋士 (a professional shogi or go player) are pronounced identically as きし but refer to completely unrelated things.
A Sino-Japanese compound written with 騎, associated with riding on horseback, and 士, a warrior or person of status; the spelling transparently reflects a mounted warrior or knight.