Most Humans cannot see or sense disembodied spirits in any way. Spirits can, however, inhabit artificial human bodies called gigai which are visible to ordinary humans. One in 50,000 humans is a medium with some awareness of nearby spirits, but only a third of these are able to see them clearly, and only the strongest of mediums are able to speak with or touch spirits. Certain unique humans naturally have both the power to sense and the strength to fight with spirits. Ordinary humans can gain the ability to interact with spirits by spending time around a large source of spirit energy. Humans are weaker then every other class but learn they're powers much more quickly. (Examples-Chad, Orihime, Ichigo's friends) They start off with 20,000 SP

Medium: A human who can See, Speak to, or Touch Spirits. Mediums can only use small amounts of Spirit Power in battles(Humans start in this form)

Spiritual Human: Gain at 50,000 SP/3, These humans have gotten used to the spirit energy revolving around them and can use they're own freely. They now can use regular Human weapons fueled by they're spirit energy. (like guns, regular swords, fist fighting, ect.)

Now they split into two classes once you get to 170,000 SP/ level 6. You can choose between "Weapon Specialist," "Priest," or "Bount." If a Human gets killed they can become a Hollow or Shinigami.