equestrian, riding on horses, counter for equestrians
騎 centers on riding a horse: the act of being on horseback, the rider, and the counter for mounted people. The equestrian sense is the core idea.
騎 combines 馬 (horse) with 奇, which likely contributes the sound キ and may suggest the idea of mounting or being astride.
A horse 馬 on the left and 奇 on the right: picture a rider sitting astride a horse, the 奇 part suggesting the rider's unusual posture.
For キ, imagine a key unlocking a stable: key -> キ, and you ride the horse out.
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(medieval) knight
riding (on horseback); mounting (a horse)
horseman; rider; jockey
counter for horsemen
horse riding; horseback riding; horseback rider
cavalry; cavalryman; trooper
cavalry
single combat; duel; one-on-one fight
single horseman
being a match for a thousand; being a mighty warrior (combatant, player)
Knights Templar
Teutonic Knights; Teutonic Order
Knights Hospitallers; Knights Hospitalers
one horseman
great warrior
outpost cavalry
the Crusaders
waiting for one tile to finish one's pair and one's hand; waiting for half of one's pair with four melds completed
military order
the Tokugawa shogunate's eighty thousand guards (comprising hatamoto and lower-ranking vassals)