The term CPU (short for 'Central Processing Unit') in the context of video games refers to elements (such as characters or enemies) whose behavior is controlled by processes programmed into the game itself. More specifically however, it is used to refer to secondary players who have been set to be controlled by the game itself, rather than any human player.
In the Kirby series, CPU players (also referred to as Computer Players, or CP in short in Japanese) can be set in any multiplayer title to stand in for real human opponents, and they are programmed to simulate how said human opponents would play to a certain degree.
CPUs often come with difficulty settings as well which the player can set to make them more or less of a challenge. The change in difficulty usually alters how quickly the CPU can react to a given situation, and whether or not they will perform certain behaviors (an example being the CPU players in Kirby Battle Royale, who are programmed to actively pick up their downed opponents and toss them into hazards at higher levels).
In every game in the series, common enemies are controlled at least to some extent by CPU programming. However, these behaviors are usually quite simple, amounting to little more than holding still, jumping in place, attacking in fixed intervals, or pursuing Kirby. CPU players - on the other hand - are typically given much more advanced algorithms and conditions which will cause them to behave in more complex manners, in an attempt to simulate strategic human play. Given the imperfect (and simplistic) nature of this programming, however, CPU players often have trouble in certain situations, and can be locked in endless behavior loops which get them nowhere.
CPU by Title
Kirby Super Star (Ultra)
The Helpers in this title are controlled by the CPU, when not controlled by a second player. They are more sophisticated than common enemies, in that they need to be able to keep up with Kirby and attack nearby threats. In special instances, they are also programmed to assist clearing certain hazards (like a Burning Leo Helper lighting a Fuse).
Kirby's Dream Land 3
In this title, Kirby can call on Gooey to help him. When not controlled by a second player, Gooey is controlled by the CPU. Like the Helpers, Gooey will attempt to stay close to Kirby and attack nearby threats. He will also swallow enemies to gain their abilities, using them to aid his arsenal.
Kirby 64: The Crystal Shards
CPU players will fill in any non-human players in the Sub-Games. The difficulty chosen for the game will affect the difficulty level of the CPU opponents.
Kirby Air Ride
In this title, CPUs play a significant role, as many objectives in the Checklists require CPU participation in some form or another. the player can set the difficulty level of the CPUs prior to a race starting, which will affect their behavior and reaction time. In Air Ride and City Trial, CPU level ranges from 1 - 9, while in Top Ride, it ranges only from 1 - 5.
CPUs are generally good at handling the road in racing modes, and can be very effective in combat situations like Destruction Derby. However, particularly in City Trial, CPUs can often be seen stuck against certain walls or obstacles (a good example being the shore just north of the Electric Lounge).
Kirby & The Amazing Mirror
When playing in single player mode, the other clones of Kirby will be controlled by the CPU. They are capable of some degree of autonomous movement, but they typically do not venture far from where they are initially placed due to generally being goalless. the player can call the CPUs to Kirby's location using the Cell Phone, where they will automatically attack nearby opponents. When Kirby initiates a Super Inhale, the CPU Kirbys are programmed to recognize this, drop their abilities and join in inhaling.
Kirby: Triple Deluxe
In the Kirby Fighters Sub-Game, (and subsequently in Kirby Fighters Deluxe) non-human controlled opponents are controlled by the CPU, with the chosen difficulty setting determining their behavior. They are programmed to attack their opponents, attempt to dodge when being attacked, and go for items that appear.
Kirby: Planet Robobot
In the Team Kirby Clash Sub-Game, (and subsequently in Team Kirby Clash Deluxe) non-human controlled teammates are controlled by the CPU. They will attempt to fight the enemy, pick up Team Meteor pieces and Food, and revive fallen comrades. They will also attempt to dodge certain attacks which the enemy throws out.
Kirby Battle Royale
CPU opponents can be set in Battle Mode, and are also encountered in Dedede's Cake Royale. In Battle Mode, their difficulty can be set between levels 1 - 5. Each different battle mode necessitates different behaviors on part of the CPU opponents, but higher levels generally involve becoming more aggressive.
Kirby Star Allies
The Friend Helpers and Dream Friends which Kirby can recruit can be controlled by the CPU, in addition to human actors. When controlled by a CPU, their behavior is similar to how helpers operate in Kirby Super Star, though they will generally prefer to keep behind Kirby. When Kirby requests a Friend Ability combo, any compatible friend will be alerted to this, and use their ability. If no compatible friend is present, they will simply display question marks (?) instead and do nothing. In addition, Friends will interact with objects in specific ways in response to Kirby's actions. An example of this is when Kirby hops in a fuse cannon - a friend with a fire-type move will be prompted to go and light the fuse.
The sophistication of the CPU friends in Star Allies is notable, as they can often be seen attempting to avoid hazards and have decent pathfinding. However, in complicated environments, they can still have difficulty maneuvering, and in some cases, may end up avoiding one hazard only to run into another repeatedly.