Skills and Arsenals can only be used by people who are called Espers.
Arsenal construction is one of the most involving parts of the game. An arsenal contains 30 slots, which are filled with skills and "Aura Particles." Skills are abilities you use in your fight against the enemy(s). Aura particles raise your Level during the fight, which allow you to generate Aura. Aura is spent to use the skills you pull from your arsenal. The selection of skills is impressive; there are 374 skills to choose from (including the 74 skills that can only be obtained through XBox Live matches) spanning 5 different "schools": Psycho, Optical, Nature, Ki, and Faith. Each arsenal can hold only a certain number of schools (1, 2, or 3), so you must be careful in choosing the skills for your arsenal. If this wasn't complicated enough, there are 6 different types of skills contained in each school: Attack, Defense, Erase, Status, Special, and Environment. Attack skills allow you to damage your target. Defense skills allow you to defend against skills used on you. Erase skills allow you to erase your target's (and sometimes, everybody's) skills, level, or even arsenal capsules. Status skills affect you or your target's status, doing anything from raising your movement speed to making your enemy's attacks do less damage. Special skills have effects which do not fall in any other skill type. Environment skills generate an Environment "crystal," which exerts a special effect on everybody until the crystal is removed by somebody, or somebody's erase skill.