Its story takes place in the distant land of Re:Eden, a world being overlooked by Heaven that is recovering from post-nuclear war, however, after the war, there has been strife between the land and Heaven as the human race began fighting against the heavenly kingdom above. Players assumes the role of the titular Karous, a girl who lost her family during the great war of the humans and seeks revenge against them, but unknown to her, she carries the blood of both humans and a goddess, carrying a connection between the two worlds.
The game is a spiritual successor to Radirgy, featuring similar gameplay elements, cel-shaded visuals, story beats, and use of subliminal imagery to its predecessor. However, unlike Radirgy, Karous is a revenge story set in a dark dysptian world with muted colors. The game also uses RPG Elements where Karous's ship can increase in power by using its shots, sword, or shield, and increasing their strengths to certain levels can unlock new abilities such as a wider spread shot with its main weapon, a 3-chain combo with the sword, or converting enemy bullets as shockwaves with the shield.
It was later re-released along with Chaos Field and Radirgy as part of the MileStone Shooting Collection: Karous Wii in 2008 (later localized in North America by UFO Entertainment as Ultimate Shooting Collection in 2009), MileStone Shooting Collection 2 in 2010 with the addition of Radirgy Noa and Illvelo, and the Sakura Flamingo Archives. for the Xbox 360 in 2014. The game received a follow-up game in the form of Karous: The Beast of Re:Eden for the Nintendo 3DS in 2015.
Karous features examples of:
- Absurdly High Level Cap: The maximum level the player can attain for each of their weapons is 100, which combined can added up to 300. Most players will more than likely reach around a total of Level 200 on Normal by the end of the game.
- Action Girl: Karous, who takes it upon herself to fight back against the human race to avenge her family.
- Animal Motifs: Of ravens. The game's title is derived from "karasu" (カラス), the Japanese word for "raven", raven feathers can be seen in the box art, title screen, menus, and the game's logo, and Karous's ship resembles a raven.
- Auto-Save: Some of the game's home console ports feature an auto-save system, which is off by default save for the Wii version in MileStone Shooting Collection 2.
- Bloodier and Gorier: Unlike MileStone’s previous shoot 'em up games and the ones that followed after, Karous features blood splats in the opening cinematic and some enemies can bleed when shot or cut with the sword.
- Bullet Hell: Similarly to MileStone's other shoot 'em ups, enemies can litter the screen with bullets. Thankfully, players have a shield to ward off bullets and the D.F.S. System to absorb them for points.
- Cel Shading: The game uses this technique to achieve anime-styled visuals like its spiritual predecessor, but rather than use bright colors, Karous uses dull and muted colors to create a somber and bleak atmosphere.
- Compilation Re-release: The game was bundled with MileStone's other shoot 'em up titles in later compilations.
- Continuing is Painful: Using credits after losing all of your HP will wipe out your previous score.
- Cool Sword: Karous's ship is armed with one, and it can perform combo attacks when it becomes stronger.
- Crapsack World: The land of Re:Eden has been ruined by a previous nuclear war and the ongoing fight between Earth and Heaven and Karous's Roaring Rampage of Revenge isn't making it any better.
- Darker and Edgier: Compared to Radirgy, the game's setting is bleak and ravaged by war. The story also focuses on Karous taking revenge against humans after they ended up killing her parents during their war against Heaven.
- Every 10,000 Points: Players can earn extends to their HP Gauge by scoring a certain amount of points. By default, the first extend is gained from 50,000,000 points, then a second extend for earning 200,000,000. The home points also offers different extend options, or turn them off entirely.
- Expy: Shigi is one to Aita from Radirgy. Both characters are friends with the protagonists who serve as Mission Control for them and wear opaque glasses that hides their eyes.
- Half-Human Hybrid: Unknown to Karous, she is half goddess on her mother's side and half human on her father's side according to the manual.
- Difficulty Levels: The standard Easy, Normal, and Hard levels.
- Life Meter: The HP Gauge, represented by the horizontal yellow/green bar. However, each hit Karous takes brings her HP down similarly to a lives system in other shoot 'em ups.
- Little Miss Badass: The game's manual mentions that Karous is 13 years old and she's fighting against legions of mechanized enemies as she wages her own personal war against the human race.
- Luckily, My Shield Will Protect Me: Similarly to the Kojiro from Radirgy, Karous's ship can use a shield to block enemy shots and - when it is leveled up enough - can also turn them into energy waves to attack opponents.
- Mana Meter: The SP Gauge, represented by the vertical blue and it is used for her D.F.S. System attack.
- One-Man Army: Karous is one, who fights her way through hundreds of mechanized enemies on her own.
- Protagonist Title: The game's title is named after Karous, the game's protagonist.
- Regional Bonus: The Japanese release of MileStone Shooting Collection: Karous Wii requires completing the titular Karous at least once to unlock Chaos Field and Radirgy. Its Western localized release, Ultimate Shooting Collection, already has both of those games unlocked.
- Roaring Rampage of Revenge: Karous will stop at nothing to take revenge against the entire human race for killing her parents.
- RPG Elements: Karous's ship can become stronger by leveling up her shots, sword, and shield by using them against her enemies, making her weapons stronger as the game progresses. Using the D.F.S. System will also yield more experience points while it's active.
- Shoot the Bullet: Some enemy shots can be destroyed by simply shooting back at them.
- Super Title 64 Advance: The Ultimate Shooting Collection was originally titled MileStone Shooting Collection: Karous Wii in Japan, with the game's packaging and menus being based on Karous.
- Smart Bomb: The D.F.S. System attack is functionally one as it clears bullets around the player in a pinch and rewards extra experience points when deployed.
- Spiritual Antithesis: Karous's story and setting is the opposite of Radirgy's; whereas the latter uses brightly colored visuals and features a light-hearted story, the former is a revenge story that takes place in a bleak world being ravaged by war and uses darker colors despite also using cel-shaded graphics.
- Spread Shot: The ship Karous uses fires a frontal shot that increases in spread and power as it levels up.
- Timed Mission: Boss battles have an arbitrary timer that is not seen in-game, and when time is up, the boss battle ends prematurely and no score is rewarded.
- True Final Boss: One can be fought only on Hard mode and by collecting the secret MileStone logo tokens hidden throughout each stage then beating the final boss before the timer runs out.
- Voice with an Internet Connection: Similarly to Radirgy, Karous receives voice-over transmissions from her friend Shigi and other characters, as well as random messages from the people on Earth.
- You Killed My Father: Karous's revenge motivation against the human race after losing her family in the great war.
