All the music for this game was composed by the Japanese artist, Revo, a man who also fronts the bands Sound Horizon and Linked Horizon. Both groups are known for lush, symphonic arrangements with rock or progressive rock elements. Linked Horizon is credit with working on the soundtrack for the game which could easily account for music such as this:
This piece is entitled "Love's Vagrant" and is the special battle theme for one of the game's characters, Ringabel. The music plays during any point in time that you activate his special abilities in battle and his overall strength is increased during the course of the song.
The first thing you hear is a burst of strings and drums that segue into an energetic theme led by an accordion of all instruments. This energetic segment continues into a mellower middle where the majority of the instrumentation outside of the concertina is reduced, allowing the bellowing instrument to take the lead. After this the song goes back into the previous, boisterous sound to close things out.
It's a short, lively piece, very befitting it's use in the game. It's not often you get to hear an accordion used effectively in modern music, let alone a video game, but the entire reason the song works so well and is so memorable is because of the instrument. Naturally, the strings and rhythm help with this too, but the distinct sound of the accordion is special unto itself. The song itself is actually an edited down version of a full, vocalized piece that you can hear in a live setting here:
The entire soundtrack comes strongly recommended due to its symphonic rock sound, but "Love's Vagrant" is a wonderful selection from it.