Short answers, pulled from the story.
Gensō Suikogaiden is a two-volume series of spin-off games from the Suikoden role-playing game franchise, released only in Japan. Both volumes are visual novels rather than role-playing games. Volume 1 arrived in the autumn of 1999 and Volume 2 followed in the spring of 2001.
Nash Latkje is the protagonist of both Gensō Suikogaiden volumes. He is an agent of Holy Harmonia's Southern Border Defence Force, assigned to investigate a rumor about one of the 27 True Runes appearing in the Jowston City States. Nash also appears as a character in Suikoden III.
Gensō Suikogaiden was released only in Japan and was never officially translated into any other language. Fan translations of both volumes were released in August and September 2013 by a group of Suikoden fans led by RinUzuki.
Volume 2 of Gensō Suikogaiden arrived roughly a year before Suikoden III and presumably helped introduce the Grasslands, which became Suikoden III's main setting. Nash Latkje, the protagonist of Gensō Suikogaiden, also appears in Suikoden III. For players outside Japan, Suikoden III was their first encounter with Nash.
Fan translations of both Gensō Suikogaiden volumes were released to the public in August and September 2013 by a group of Suikoden fans led by RinUzuki. No official translation has been produced.
The Gensō Suikogaiden games were made by a separate development group at Konami. After completing Volume 2, the Suikogaiden team merged with the Suikoden III team. The combined group then began work on both Suikoden IV and Suikoden V.