PSI or PK are the powerful psychic abilities of the Mother series. It is attached to both offensive and defensive moves, among other useful abilities.
PSI was first used by the aliens, such as Giygas' race. George, a human abducted by those aliens, conducted research on PSI, and discovered that some humans with the right potential are able to use PSI, such as his great-grandson Ninten.
PSI is rarely used in the world of the Mother games. A few enemies, and even less playable characters are capable of using it. There are about six characters that can be used through normal gameplay that are capable of using PSI, and each was born with the potential to use it. These characters are not related, and are also from different worlds, and yet they have the ability to use PSI from various means.
PSI has a limit to how much PSI you can use. This is called PP, or Psychic Points. Each PSI spell costs a certain amount of PP. The more effective a spell is, such as damage or healing, the more PP it costs. Of course, there is a way to raise PP with an item, or even with other PSI.
PP can be raised in two different ways: eating a magic food, or using PSI Magnet. Magnet usually takes a small amount of PP, such as 2-10 PP, but it costs no PP to take another's PP. Eating a magic food item, such as a Magic Gelatin, raises PP by far more. However, these magic foods aren't easily found. They are usually one of the rarest items to find in the field.
PSI spells have five levels: Alpha (α), Beta (β), Gamma (γ), Sigma (Σ), Pi (π), and Omega (Ω). The effectiveness of a spell is ranked from lowest (Alpha) to highest (Omega), with Omega being the most efficient, as well as the most costly. Some spells go from Alpha to Omega, minus Sigma and Pi, while others only have Alpha and Omega.
PSI ranges from hitting one enemy, to them all. The same goes with allies, although usually with stat-raising and healing. Although enemies can also utilize harmful PSI. Alpha and Beta spells usually only harm one enemy/ally, while Gamma, Sigma and Omega spells usually affect all enemies or allies.
There are four kinds of PSI: Offensive, Healing, Assist, and Other. Offensive, of course, causes damage to the enemy. Healing heals allies, while Assist either raises or lowers stats, as well as causes status effects. Other is for any PSI that doesn't fit any of these categories, such as Telepathy.