Ikaruga is a top-down shoot-em-up produced and scored by Iuchi Hiroshi of Treasure (Developer).
It's considered the spritual successor to Radiant Silvergun, despite its minimalist ruleset. The key feature is the duality system where you can switch between white and black and absorb the bullets of your current colour.
The Sega Dreamcast version has a backstory about some guy who got shot down and the assassin sent to kill him who joined his cause.