See them, no. Most spirits are invisible to human eyes.

And even then not all humans are capable of forming contracts with spirits. It requires a human with a mana lobe. [She points to her head, indicating where such a thing would be located.] By offering the spirits mana an exchange is made. The spirit receives mana and the human receives the spirit's power.

Humans who cannot have created devices known as Spyrix to help them harness that power. However, they were doing so at the cost of the spirit's lives.

Right now they are attempting to build a safe method to do that, which won't endanger the spirits.