Story:Handkerchief, developed Opus Corp. / Ask, Co., Ltd., published by XSIV Games (Stealth Media Group).
Side-scrolling run'n'gun similar to Metal Slug or Jazz Jackrabbit. Shoot, jump, use weapons and vehicles to combat the enemy army. Single or multiplayer.
Played it as a kid and it remained stuck in my mind ever since. It has great soundtrack and interesting gameplay, where you convert enemies to your side and then switch between them to use their weapons.
I've played a demo of this when I was a kid. The art style and gameplay made a great impression on me that I remember the game 'til this day. For a very long time I didn't know the name of this game until few months back when I stumbled upon it on a random website.