This is “Azalea Town” from RPG “Pokémon HeartGold/SoulSilver”(2010), developed by Game Freak, published by The Pokémon Company.
This is originally composed by Go Ichinose(Twitter), who has participated in composing music since “Pokémon Pinball”(1999) as for the series,
and arranged by Shota Kageyama(Twitter).
Go Ichinose is known as a horse racing enthusiast, and is one of the planners (and composer) of “Pocket Card Jockey“(2013), also developed by Game Freak.
The game was produced as a remake of “Pokémon Gold and Silver”(2000).
Compared to the original, for example, the trainer’s strategy has improved and increased in difficulty.
In addition, there is now dedicated music for the battles against the legendary Pokémon, Ho-Oh and Lugia.
Compared to its previous title, “Pokémon Diamond/Pearl“(2007), there is additional features such as the possibility to take the first Pokémon in your possession out of a Pokéball and carry it with you.
Azalea Town is where the second gym is located, and the protagonist first encounters and fights Team Rocket at Slowpoke Well.
I think the appeal of this song is that it somehow evokes a sense of nostalgia.
This is especially felt in bars 12th and 20th (both with the same chord progression).
The chord progression is iii-III-vi. III-vi has a strong relative minor feeling. I think that the sudden darkening of the bright tune gives a sense of nostalgia.
In bar 20th, 3rd of the chord appears in the melody, so it is more easily recognized.
After that, the movement of the root of vi down a semitone is also popular.
(As far as I have observed, ZUN-san of the Touhou series frequently uses such a way, in which the root moves down a semitone from the tonal center chord of the minor key, in the music for the boss on the 6th level.)