So... is Ribbon no Kishi finished or not?
It is a bit ambiguous in this case.
Why?
The anime finished after the manga:
Anime finished: April 07, 1968
Manga finished: September 03, 1966
... But we could not confirm the source content in MangaUpdates.
Watching Order (by release date)
Here you have all the anime relations we detected ordered by release date, so you can see the reason for this status in context and also use it as watching order.
The mangas and the non-core contents are excluded by default, but you can enable them by clicking the buttons below:
Ribbon no Kishi Pilot
リボンの騎士（パイロット）
Anime - Prequel
Release date: November 28, 1966
Ribbon no Kishi
Princess Knight
Anime - Base
Release date: April 02, 1967
Synopsis
In the kingdom of Silverland, a prince named Sapphire hides a profound secret: they are biologically female. Circumstances at birth led to Sapphire possessing both male and female hearts, a secret that must be guarded to prevent Duke Duralumin from seizing power, as a woman cannot rule.
To correct this anomaly, the angel-in-training Tink is dispatched to retrieve Sapphire's male heart, but faces resistance. Amidst the Duke's machinations, Sapphire struggles with their dual identity, especially when romantic feelings develop for Prince Franz Charming from a neighboring kingdom.
Thus, Sapphire and Tink embark on a series of adventures, navigating treacherous situations and battling foes to protect the throne and maintain the delicate balance of Sapphire's secret.