This question stems from the name of the game: Ni No Kuni which in Japanese is 二ノ国.
I'm not sure what the intended meaning is here.
The particle の connects two nouns, where one modifies the other.
While it is true that in many instances this denotes a relation of possession (genitive):
AのB。"B belongs to A"
私の名前 "My name"
it is not always the case, and の may indicate any relationship or connection between both nouns arbitrarily. The nature of this relationship is determined by context, like many other aspects in Japanese:
司会のミラーさん。"Moderator Miller" (A is qualifying or describing B)
男の人。 "A man" (Again A is qualifying B)
Note that among learners of Japanese as a foreign language, there's a whole category of the so called "の adjectives", where an "adjective" modifies a noun connected by the の particle. From the standpoint of Japanese grammar, there's no such category and the supposed "の adjectives" are simply nouns, that happen to modify other nouns by connecting both with の.
To answer your first question, "Is this still the particle の ?" the answer is yes, ノ in 二ノ国 corresponds to the particle の without a doubt.
I found this entry in the sister site anime.stackexchange.com where they try to determine the meaning of the title. The accepted answer states that:
So the final title is either: "The Country of Two" [...] or "Second Country"
I checked the entry for 二ノ国 at the Japanese Wikipedia. It turns out that besides 二ノ国, there is also an 一ノ国. Since there are actually 2 countries, it only makes sense to translate 二ノ国 as "Second Country" or "Country number two" rather than "The country of two":
シズクは現実世界（『一ノ国』）とは異なる並行世界二ノ国からやってきたのだという。一ノ国と二ノ国はそこに暮らすものの魂が繋がる表裏一体の世界だった。
Shizuku is said to have come from a parallel world called Ni no Kuni, which is different from the real world ("Ichi no Kuni"). Ichi no Kuni and Ni no Kuni were two sides of the same world where the souls of those who lived there were connected.
Please note that 二ノ国 is not the common way to count stuff in Japanese. Usually, to count objects in Japanese you should use counters. This is a complex topic and I can't explain it here, just beware that numberのnoun is not the only nor the standard way to count things in Japanese. Here you are some examples of numbers combined with nouns (counters in bold font):
三{み}つのミカン Three mandarines (general counter)
三{さん}番{ばん}目{め} の車 The car in the 3rd position (ordinal counter)
第{だい}3課{か} The 3rd lesson (counter for lessons)
三{さん}月{がつ} The 3rd month, March (counter for months)
三{さん}ヵ月{げつ} Three months (counter for months)
There is a counter for countries, with the same kanji 国, but it is pronounced 国{こく} instead of 国{くに}. However, this counter does not seem to be used regularly (see the discussion in the comments section):
三{さん}国{ごく} Three countries (counter for countries).
Well, the quick and easy answer, it is just being used as katakana to be pretty.
No actual reason.
Like in the pic:
