just to point out that in this novel there are two separate noble ranks, you have the feudal landholding ranks where Leon is still a Baron at this point in the manga and then you have the royal court rank which is more of a political ranking unrelated to landholdings but involved with government work and how "close" you are with the royal family. historically there has been bureaucrat nobility without lands that served in administration while receiving a "stipend" related to their court rank
"Eighth and ninth ranks are for those who have been knights their whole life. One can enter the seventh rank from heritage, but ascending one rank above that would require many years of loyalty and some large achievements." And by many years they mean like 3 generations. It's pretty much unheard of for Leon to get this promotion.
Then @Huapollon explained the likely inspiration from Imperial China:
Its based on the Japanese/Chinese system of nobility, where there is a "peerage" and a "court rank".
Classically, Court rank usually goes from one to nine, with each rank split in two (for a total of 18 ranks), with the senior first rank being the highest, and the junior ninth rank being the lowest...
Edit: Cause some people still misinterpreted, have you read Kimetsu no Yaiba? Yes? Okay 12 Moons, Upper 1-6 and Lower 1-6 right? Leon was a lower and jumped to Upper 6 (other readers pointed out it's not "risen above 6th" but "upper 6th") out of Upper 1-9 and Lower 1-9. If you haven't read Kimetsu no Yaiba and have no idea what I'm talking about I still hope this helps.
There's multiple peerage/rank systems in Europe, some overlap some don't. I just copied the wikipedia condensed version not accounting for differences between systems. However the modern UK system and Imperial Japanese system have been included afterward for comparison and distinction.
TL;DR version From highest to lowest:
Emperor
High King
King
Archduke
Grand Prince/ Grand Duke
Prince/Prince-elector/Crown Prince
Duke
Sovereign Prince
Marquess/Margrave
Count/Earl
Baron
Baronet
Eques/Knight/Lord
Gentleman/Gentry
Modern UK Peerage system
King
Duke
Marquess
Earl
Viscount
Baron
Imperial Japanese prior to end of WW2
Koshaku/Prince = Duke
Koshaku = Marquess
Hakushaku/Earl = Count
Shishaku = Viscount
Danshaku = Baron
So it'd seem most isekai with European settings use a mix of German, HRE style ranks and British ones, the most common hierarchy I've found being
King
Duke
Marquess
Earl
Count
Baron
Baronet
Knight/Lord
@SunSun Viscount was mentioned earlier and is most likely the next rank directly above Baron. The other previously mentioned ranks have been independent knight, baronet, marquess, earl, duke, crown prince, and it's safe to assume there is a monarch. I think you could also say Colin and the other younger siblings are classified as gentry with no peerage or lands of their own.
Only manga/light novel where getting a higher rank comes with more demerits than positives.
Higher rank means more girls want you, but in that world, the girls buy demi-human slaves that don't reproduce so they expect you to take care of them while they have lovers on the side. Most girls are legitimately shit, including the protagonist's blood sister. Not to mention all the responsibilities that a higher rank comes with. You're meant to have a military force and the higher your rank the more likely you are to get deployed on missions by that faulty government.
Being a commoner is legit ass as you're stuck in manual labor hell too.
Best sweetspot is lower noble. You don't have too strong of a pressure to get married, and nobody has expectations of you so you have ALL of the freedom as well.
Honestly in the narrative, him not thinking he can date Angie makes perfect sense, but I never understood why he didn't pursue Olivia... OH yeah now I remember. He still thinks that because Olivia's the protagonist she's going to wind up in some super high position like being married to the prince still or something.