A Room Guarder is a type of mid-boss in the Kirby series that first appeared in Kirby's Adventure and was popularized in Kirby 64: The Crystal Shards. They generally consist of many regular enemies in a room/area that cannot be exited until they are all defeated.
Room Guarders make their first appearance in Kirby's Adventure as the Meta-Knights. They are the only type of Room Guarder in the game, and appear in every level except Vegetable Valley and Rainbow Resort.
In Kirby's Adventure, the boss theme plays; in Kirby: Nightmare in Dream Land, the Room Guarder theme from Kirby 64: The Crystal Shards plays. The reason for the music change is likely because Kirby 64: The Crystal Shards was the first game to have its mid-bosses be exclusively Room Guarders.
Like in Kirby's Adventure, the Meta-Knights are the only Room Guarder mid-bosses in the game. They appear numerous times in the sub-game Revenge of Meta Knight, but have also appeared in Dyna Blade's Mallow Castle and The Great Cave Offensive's Old Tower. Usually only two are actively fighting in Dyna Blade and The Great Cave Offensive, while in Revenge of Meta Knight, usually three are actively fighting. The Meta-Knights are also occasionally just regular enemies, only occurring in Revenge of Meta Knight, Aquarius in Milky Way Wishes, and in the fight against Galacta Knight (however, all three Meta-Knights summoned must be defeated in order to advance the fight, similarly to a Room Guarder).
All of the mid-bosses in this game (except for Waddle Doo, Adeleine, and King Dedede) are Room Guarders, however, most of them feature a large version of a regular enemy. Regular-sized enemies, usually of the same type as the large one, can be used as ammo if Kirby does not have a Copy Ability; they respawn after being defeated. Once the large enemy is defeated, the regular enemies can be defeated without respawning, and Kirby is allowed to exit the room. Some of them also drop a Crystal Shard for Kirby to collect. In the fourth stage of Neo Star, there is a Room Guarder that is simply a bunch of Drops falling from the ceiling. Stage 3 of Ripple Star also features Room Guarders that are various regular enemies without a large one in the room. This area serves as a Mid-Boss Tower. This game was the origin of the Room Guarder theme, which is a remix of the mid-boss theme; the theme would play in later games that have Room Guarders, such as Kirby: Nightmare in Dream Land, Kirby: Canvas Curse and Kirby and the Rainbow Curse, as well as against mid-bosses in Kirby: Squeak Squad and Kirby Mass Attack. The theme is also remixed in Kirby Air Ride as an alternate theme for Light course in Top Ride, and in Kirby Star Allies as a theme used for various puzzle rooms.
All of the mid-bosses in this game are also Room Guarders, however, they have much less variety of enemies, although are generally more unique than those found in Kirby 64: The Crystal Shards. The Room Guarders consist of Spear Waddle Dees, Inflatable Bombs, Parachute Bombs, and Thudds. The Inflatable and Parachute Bombs can only be defeated by tapping on them, whereas the Spear Waddle Dees can be defeated through traditional means; Thudds must be defeated by passing under them while over a pit.
The Squeakers act as Room Guarders in some of the Squeaks' hideouts in the game. Other times, the hideout simply houses a few Bubble items. If the main Squeak makes it to the hideout, Kirby will need to defeat him in addition to all of the Squeakers.
Room Guarders reappear, this time featuring a wider variety of enemies. Room Guarders often start out easy and progress in difficulty as Kirby and his friends defeat more enemies; this difficulty curve may take the form of spawning either larger, more numerous, or more aggressive enemies. Room Guarders often appear successively, each one more difficult than the last. Occasionally, a Room Guarder will contain a mid-boss, either at the end or as a midpoint of the battle.