"Ace" is a term for a skilled fighter pilot dating back to the dawn of air combat in World War I and is now generally regarded as designating a pilot who is responsible for downing 5 (or more!) enemy aircraft.

During World War I, French military pilot Adolphe Pégoud, who'd been a well-known daredevil pilot before the war, was described in French newspapers as "l'as" or in English "the ace." This is a reference to card games in which the ace card often has a high value or special power. The term "ace" was subsequently applied to other outstanding pilots.

As a general matter, the number of victories achieved by a pilot was tracked, and after a certain number (eventually 5 in many air forces though in some case 10) it became customary to award a medal, which would be advertised in the press. At that point the new ace became known and would often receive further publicity. These aces were consciously compared to sports heroes and the "knights of old" and developed fans. They also competed with each other. This glamorization of individual aviators was both a continuation of the prewar fascination with flight and a contrast to the anonymous horrors of mass industrialized warfare on the ground.

It's notable that, because aces were publicized through the civilian press, the enemy military and public became aware of aces. This had a dual aspect. On the one hand, it was a great coup to defeat an enemy ace. But on the other, many pilots and fans recognized the enemy aces were not so different from the friendly aces, so at times expressions of sympathy were made when an enemy ace was killed. This started at the very beginning of the "ace" era. When Pégoud was killed in 1915, the German pilot who had shot him down flew over his airbase and dropped a wreath. The German press published sympathetic articles, which the French press in turn summarized for its readers. A French pilot later shot down that German pilot and was celebrated as having avenged Pégoud. Things like this still happened in World War II, though they were rare.

The Red Baron (Manfred von Richtofen) is probably the paramount example of an ace. Note that he was a historical person, regarded as the highest-scoring ace (or "ace-of-aces") of WWI. The question of who shot him down is still debated. He later became a kind of legendary figure, found in popular songs, comic strips, and as the name of a brand of pizza. But he was quite real!

Anyway, WWI ace culture really never went away and was picked up again in later wars. It was part of how the public expected wars to be discussed and part of the internal culture of the air forces.

So the Ghost of Kyiv legend follows pretty naturally from these antecedents, albeit most likely originating in online forum and memecrafting cultures rather than formal journalism and military awards.