@SunSun Sorry, have never read that series you mentioned. As for the ranking, it is pretty standard knowledge across East Asian countries.
To satisfy your curiosity, in imperial china, there are actually two different court rank variations: One called 9 classes of 18 tiers, another called 9 classes of 30 tiers.
9 classes of 30 tiers (Which from the 4th class down have a "upper" and "lower" variation, aka. Senior 4th Class Upper) was implemented in the Sui Dynasty and was in use through the Tang and early Song dynasty, when one of the Emperor in the Song Dynasty simplified it into the 9 classes of 18 tiers (Removing the upper and lower variations). The ranks are primarily tied to court (in other words, government) positions, with the Prime Minister of the emperor rated as the Senior 1st Rank. (Additionally, the ranks are directly tied to your wage and benefits - including how many horses you can pulling your carriage or how big a doorway and house you can build for your residence, similar to military ranks today)
18 tiers: https://zh.wikipedia.org/wiki/%E4%B9%9D%E5%93%81%E5%8D%81%E5%85%AB%E9%9A%8E
30 tiers: https://zh.wikipedia.org/wiki/%E4%B9%9D%E5%93%81%E4%B8%89%E5%8D%81%E9%9A%8E
In other words, they are ranks of civil servants. If it were to be in use today, the lady at the DMV counter would likely be counted as Junior 9th Rank.
As the Japanese Imperial Court largely copied the Tang Dynasty Court wholesale, they largely retained the 9 classes of 30 tiers version before it was phased out.
https://en.wikipedia.org/wiki/List_of_Japanese_court_ranks,_positions_and_hereditary_titles
p.s. In case you are wondering, Kingdom of Korea, Vietnam, and Ryukyu all adopted the 9 ranks of 18 tiers system, only Kingdom of Japan kept using the 9 ranks of 30 tiers system.
Unlike in Imperial China, where governmental positions were selected by merit and tied to civil service, the Japanese court positions largely evolved to became hereditary, and by the beginning of Shogunates, devoid of any legitimate power. (aka. a Minister of War having no armies to command, or a Minister of Revenue with no taxes to collect.) As this is a japanese manga, they largely inherited their interpretation of court ranks - them being powerless honorary positions - although using the simpler 9 ranks of 18 tiers system.
As for Peerage. Historically, there are technically only 3 ranks of nobility:
Emperor/Independent Kings (With Emperor STRICTLY limited in use for line of the ROMAN emperors. As such, classically, there are only 2 legitimate emperors in europe, one for eastern roman emperor - claimed by Russia after Byzantine Empire fell in form of Tsar, and western roman emperor - the Holy Roman Emperor. Britain's Emperor/Empress is actually for the Mughal Empire, not for anywhere in Europe, and Napoleon is just a trend breaker.)
King/Prince (aka. Fursts)/Margrave/Duke/Count/Baron
Baronet/Knight
Which term you end up using is largely depending on what that plot of land you own is called historically, and how full of it you are. (Like Dukes of Austria calling themselves Archdukes one lovely morning, or Principality of Hanover suddenly being called the Kingdom of Hanover because they feel like they deserve a little more recognition.)
Baronet and Knights are the only unique ones, since they are not tied to land, and are the only ranks who are considered subservient to another rank of nobility.
Lastly, as a translator, I can say the "use a mix of German, HRE style ranks and British ones" is an artifact of translation, in other words, it is that we use whatever we feel most comfortable with. Some of us prefer to translate 伯爵 as Earl (I'm staring at you, my silly british translator friends) while others prefer using the (more proper) term of Count, and that ONE silly translator who for some inexplicable reason, always use the term Comites.
As such, ALL ranks in japanese literature uses the Japanese peerage system, itself inherited from Chinese Imperial dynasties since the Zhou Dynasty (As with everything else, introduced wholesale into japan during the Tang Dynasty). Any variation in term is solely discretionary by the translator in question.
https://en.wikipedia.org/wiki/Kazoku
https://en.wikipedia.org/wiki/Chinese_nobility#Peer_ranks_of_the_Zhou_dynasty
p.s. Add a King/Emperor on top of it all, and yes, I've yet to see a japanese author understand the intricacies of using the term emperor, so they use it strictly in the persian sense of "kings of kings", aka. a ruler who rules over multiple "likely extinct" kingdoms and/or multiple races.
Last tidbit for translators: Please UNDERSTAND the difference between Regent and Regnant... I seen too many cases of translators use the term Regent when they meant Regnant. (A retired king is a "retired regnant" not "retired regent", after all, he is not serving as a regent for a capable young king.)