PK Fire (ＰＫファイヤー PK Faiyā) (called PSI Fire in the English version of EarthBound) is the side special move of both Ness and Lucas. When performed, the character fires a yellow, lightning-bolt shaped projectile, that travels in a straight line until it hits an enemy or destructible portion of a stage, at which point it explodes into flames. If it does not hit either of the aforementioned objects within a few feet or hits a wall, then it dissipates. The way the PK Fire explodes differs between the two characters.
Ness' PK Fire has been the same through the entire Smash Bros. series. The angle at which Ness would shoot PK Fire would be horizontal, although angled down a bit, on the ground, while diagonally downwards if used in mid-air. If Ness' PK Fire hits, it explodes into a stationary pillar of fire that hits his enemies several times. In mid-air, however, it falls down slowly until it hits the ground or dissipates. DI is not very effective against it, allowing Ness to combo his opponent or use one of his finishers while the enemy is stunned. Ness can use this move in conjunction with his second jump to perform the PK Jump, which slightly boosts the air speed of his second jump at the expense of height, giving him a slightly better approach. The main difference is the damage within the games and the lag/speed of the move. It does overall the most damage in Super Smash Bros. Brawl, followed by Super Smash Bros., and then followed by Super Smash Bros. Melee.
This is a simple combo often used for gimping. This is best done near a ledge. First the player must use Ness' PK Fire on an opponent. Then they must follow up with a forward throw, which will knock them off the edge. From here, they can use Ness dair spike, which will usually KO the opponent. Alternatively, the player may use d-throw after PK Fire, and if near the edge, finish with a dair spike. However, the opponent can easily DI the forward throw and avoid the dair. Still, when used in surprise, it can be an effective combo.
Ness's Down Throw in Melee and Brawl also involves PK Fire. Ness will throw his opponent to the ground, and then burn them with a scaled down PK Fire. In Melee, this will deal 8% damage overall, while in Brawl, this will hit with 5 blasts of Flame, dealing 1% damage each, and then the opponent will take one final blast, dealing 4% damage, making a total of 9% damage overall.
In Brawl, Lucas's PK Fire always travels horizontally, regardless of whether or not he is in the air. When the bolt hits an opponent, it explodes into a horizontal burst, hitting once, and dealing fair knockback to the enemy. The horizontal burst has good range, and can damage others in the way of the burst, although it does little-to-no knockback. Lucas' PK Fire has many interesting properties, such as pushing him back with a set force after shooting the bolt. Because of these Lucas can use his PK Fire to perform the Zapjump, the Wavebounce, or, an extension of the Wavebounce, the Magnet Pull, making this one of Lucas' most varied moves.
PK Fire (PSI Fire in North America) first appeared in Mother and has appeared in later Mother games since. In all three of the Mother games, the move involved using the character's psychic power to send fire at the enemy. However, in the Mother series, neither Ness nor Lucas learn the move (which is something that Paula's trophy in Melee explains as she was the one who learns PK Fire in the game). In the description for the move in EarthBound, it states that fire bursts from the player's fingertips onto a row of enemies. As stated before, despite Ness and Lucas not being able to learn PK Fire at any instant in the Mother games, Ness and Lucas can use it in the Super Smash Bros. games. Also, when Ness and Lucas perform this attack, fire comes from their fingertips, which is how the move is described in EarthBound.