PSI refers to psychic abilities used by characters in the Mother series. Both playable characters and enemies feature PSI techniques, which get stronger as the game progresses. A PSI ability may feature many levels, marked by the Greek letters alpha (α), beta (β), gamma (γ), pi (π) and omega (Ω) in Mother, with pi replaced by sigma (Σ) in EarthBound, and with Mother 3 not having it, as well as replacing gamma for delta (δ).
In the three games of the series, PSI techniques are learned by leveling up, or by a specific event, though each series introduces PSI in a different way. In Mother, the main character Ninten would learn his first PSI technique at the beginning of the game because of the appareance of a Poltergeist. This game explains PSI comes from Giygas's race. Other PSI users in the game have unknown origins. In EarthBound, the main character Ness displays psychic powers since he was a baby, as seen in a memory, so it is not connected to the alien Giygas, despite it appearing in the game. Paula and Poo can also use PSI, again in an unexplained way.
In Mother 3, PSI is related to the Magypsies. Aeolia mentions in Chapter 1 teaching a PSI technique to Claus. In addition, Lucas learns PSI with the help of Ionia in Chapter 4. Kumatora also possesses PSI, and it is later explained she was raised by the Magypsies. Learning PSI in this game, however, involves the character previously getting a fever. When it goes away, the character learns a new technique. Other extreme situations like being hit with lightning can make one learn PSI as well. In an interview with Shigesato Itoi, the creator of the series, he stated that the process seen in Mother 3 is similar to menstruation.
(Mother 3 Spoilers Ahead)
PSI Rockin' and PK Love are abilities used by Ness and Lucas only in EarthBound and Mother 3, respectively. Both PSI attacks are named after what the player writes as a favorite thing, with the default being "Rockin'" for EarthBound and "Love" for Mother 3. While they're normally considered different PSI attacks, both have the same effect of dealing a big amount of damage to all the enemies in battle, at the same time costing notable PP compared to other offensive PSI moves. While Ness learns PK Rockin' by leveling up, PK Love is more important to Mother 3's storyline. It is learned by Lucas at the same time he learns PSI, with the help of Ionia, while the three more powerful versions are learned by pulling the Needles. The only other personed able to use PK Love is the Masked Man, later discovered to be Claus, and the same PSI is used by him against Lucas, even in the final battle. It is normally thought they have both been given this power for being siblings. One PK Love attack by the Masked Man is notably responsible for damaging Flint in the end of the game.
PK Starstorm is a powerful PSI attack that hits all enemies with great power, much like PK Rockin' and PK Love. It first appeared in EarthBound, used by Poo, who learned the α version with the help of the Star Master before the party reached the Deep Darkness, and the Ω version is learned before the groups goes back in time for the final battle. In the case of Mother 3, Kumatora learns it after Lucas pulled the sixth Needle, thanks to Ionia. PK Starstorm is also famous for being both Ness and Lucas's Final Smashes in Super Smash Bros. Brawl.
PK Fire, Thunder and Freeze are PSI powers present in all three games in the series, mainly used by characters with offensive PSI, which includes all the playable girls in the series, Ana, Paula and Kumatora, and Poo, aside from enemies. These three PSI normally have an advantage against specific enemies, and may have a disadvantage as well. The Barrer Trio, for example, when doing a pose is immune to two out of these three techniques. These three techniques are used in the Super Smash Bros. series, even though none of the users possessed them in their own games.
PK Fire deals fire damage to a group of enemies, and may engulf one in flames. In Mother and Mother 3, all enemies in the battle are affected, while in EarthBound it affects one row of enemies (each enemy can be in a front or back row). Fire-based attacks that are not PSI used by enemies normally follow this pattern as well and hit all the party. In the Super Smash Bros. series, Ness and Lucas can shoot a PK Fire that makes consecutive hits to the enemy, or explodes in a big hit, respectively.
PK Thunder deals a number of hits to random enemies, and sometimes turns their bodies numb. The number of hits is based on the level of the PSI. For each thunder fired (with the highest being four), one enemy is randomly selected. It is a powerful attack, but this characteristic makes it unpredictable, as it may not hit the intended enemy, or it may even hit one immune to electric attacks and, so, waste one attack. Thunder-based attacks used by enemies also hit randomly. In EarthBound and Mother 3, the Franklin Badge is used to reflect lightning attacks, which include this PSI. In Mother, however, the badge has no effect on PK Thunder. In the Super Smash Bros. series, Ness and Lucas can shoot a PK Thunder controlled by the joystick. It can be used to hit enemies to cause damage, or it may be used to hit one's own character, which launches him and can be used for recovery or as a powerful attack.
PK Freeze is the most basic attack. It only hits one selected target, and always scores one hit only, with the possibility of solidifying an enemy. Because of this, though, it is powerful as well. Enemies weak to PK Freeze normally include those whose body is composed of fire or lava. In the Super Smash Bros. series, Lucas uses PK Freeze, controlled similarly to Ness's PK Flash.
PK Ground is an ability introduced in Mother 3, usable only by Kumatora. It is one of the strongest PSI and, so, learned at a very high level. It deals damage to all enemies equal to 2 to 15% of their max HP, and is immune to shields and counters.
Aside from PSI that damages enemies, there are numerous PSI able to help in battle in different ways, such as recovery.
Lifeup targets an ally, and can be used outside of battle as well. It makes the character recover HP. The amount is determined by the level of the PSI. Mother had the three first levels recover one single ally's HP, while the remaining two targeted all allies. It was usable by both Ninten and Ana, with the former missing the last level and the latter not knowing the first level. In EarthBound and Mother 3, the three first levels heal one ally, and the last one the entire party at once. It is learned by Ness, Lucas, Poo and Kumatora, though the latter two only learn it up to a certain level.
Healing is a PSI able to cure status ailments. In Mother and EarthBound, the levels indicated what type of status it can cure. Normally, a higher level also works for any status a lower level cures. In the former, α cures poison, β cures paralysis, γ cures stone, and π cures sleep, while Super Healing cures death. In the latter, α cures colds, sunstroke and sleep, β cures poison, nausea, uncontrollable crying and feeling strange, γ cures paralysis, diamondness and death (75% chance of working), and Ω cures death with no missing chance. In Mother 3, on the other hand, Healing cures any status (except for death unless indicated) but the number is determined by the level. α cures one for one ally, β cures two, δ cures all, including death, and Ω does the same for all the party.
PSI Magnet appears in all three games. It is used to drain a small amount of PP from the enemy. In EarthBound and Mother 3, α targets one enemy while Ω targets all enemies. In Mother, it could only target one enemy and was assigned no Greek letter. It is learned by Ana, Paula,Poo and Kumatora only.
Shield off is a PSI used only in Mother, able to break a target's shield. It is notable that in the other two games no such PSI exists, and shield breaking is limited to items.
PSI Block is, again, a PSI limited to Mother. It prevents the target from using PSI attacks. The only comparable mechanic in the other two games are disrupted senses and fleas ailments, in EarthBound and Mother 3, respectively.
Status affecting PSI
These powers have the ability of causing a status ailment to the target.
PK Flash is a PSI used by Ness and Lucas, listed in-game as Offensive PSI, even though it does not make any damage. To each enemy, it has a 60% chance of making them cry, 10% chance of paralyzing them, 10% chance of making them feel strange, 10% chance of causing instant death, and 5% chance of doing nothing, with each probability applied to an enemy's specific weakness. In Super Smash Bros. Melee and Brawl, Ness uses PK Flash as a green light that charges until it is left to explode.
Hypnosis can put a target to sleep, making it unable to attack until woken up. Paralysis turns the target's body numb, disallowing it to do physical attacks. Brainshock makes targets feel strange, making them attack allies, or use random items. In all cases, in EarthBound and Mother 3, α targets one enemy and Ω affects all the ones present. In Mother, Hypnosis and Paralysis worked for one target only, while Brainshock's multi-targeting version was called Brain Cyclone.
Only found in the original Mother, Darkness was used to blind a target, reducing his, her or its accuracy. However, in the two following games, it has been replaced by the Crying status, which is caused by PK Flash.
These PSI can increase or decrease the stat of an ally or enemy, respectively. This only works for the remainder of the battle, however.
Offense up increases the offense of allies, and Offense down reduces it for an enemy. Offense up was used in Mother by Ninten, by Paula in EarthBound, and by Lucas in Mother 3. Offense down only appeared in Mother 3, usable by Kumatora.
Defense up raises the defense of one ally, while Defense down decreases the defense of an enemy. Ninten used both in Mother. Paula used Defense up in EarthBound, and Lucas used it in Mother 3. Defense down was used by Kumatora in Mother 3.
Quick up only appeared in Mother, where Ninten used it to boost an ally's speed.
Shields in the Mother series may be difficult to categorize because of their different terminology. In general, there are four types: those who reduce physical damage, those who do so and also reflect the damage taken, and the two equivalents for PSI damage. Shields may break after a certain amount of hits one character takes. On the other hand, if one character has one shield and gains a new one, it will become stronger, but it only can be done once.
In Mother, PSI Shield protected from both physical and PSI damage, with α targeting one ally only and β the entire party. Power Shield did the same effect, and also reflected the damage. This one only existed in one version that targeted one ally.
In EarthBound, Shield takes care of reducing physical damage, while PSI Shield does so for PSI damage. In both cases, α targets one ally, Σ targets all the party, β targets one ally and reflects his or her damage, and Ω does the same for everyone.
In Mother 3, Shield reduces physical damage, PSI Shield reduces PSI damage, Counter reduces and reflects physical damage, and PSI Counter does the same for PSI damage. In all cases, α targets one ally and Ω affects all the party.
Teleport works only outside of battle. It permits the characters to instantly travel to another place in the world previously visited. It is needed that first the characters run at a fast speed without stopping for it to work. Ninten and Ana learn it through a baby from Youngtown, while Ness learns it from a monkey from the Monkey Caves in the Dusty Dunes Desert.
Telepathy is a PSI used in Mother, where it can read non-playable characters' minds and objects in order to find information.