There are 6 types of character Nexus in game: Animal, Artificial, Human, Inorganic, Magic and Vegetal.
The animal nexus is used in summoning. The higher the nexus, the better animals you can summon.
The artificial nexus is used in manufacturing. You can't make high level items without a big artificial nexus.
The human nexus limits what kind of equipment (armor, weapons, etc.) you can wear. Higher nexus, better equipment.
The inorganic (or mineral) nexus is used primarily for harvesting and some alchemy purposes.
The vegetal nexus is used in potions. The higher, the better.
Below is the list of creatures & items that require the Human nexus.