Spirits are the intelligent beings created by God. They are the individualisation of the intelligent principle and populate the universe. They are quintessentialised matter, so ethereal that they could not be perceived by our senses.
There are different orders of Spirits, according to the degree of perfection they have attained. The Spirits who have attained maximum perfection are in the first order: they are pure spirits. Those who have reached the middle of the scale are in the second order. They predominantly have the desire to do good, they are the good Spirits. Those upon whom matter predominates over the spirit are in the third order and at the bottom of the scale: they are imperfect spirits. Ignorance, the desire to do evil and all their evil passions retard their progress.
[Taken from questions 76, 82, 96 and 97 in the sections Origin and Nature of Spirits and Different orders of Spirits from the 2nd book – Chapter I of The Spirits' Book by Allan Kardec]
Are Spirits good or bad by nature?
God has created all Spirits simple and ignorant. They progressively reach perfection by getting to know the truth and through experiencing trials. Had they been created perfect, they would not have any merit. They have to go through lengthy experiences. Their free will develop as they acquire consciousness of themselves. They undergo influences and must develop their insight in order to make the best choices.
[Taken from questions 114 to 127 in the section The progress of Spirits from the 2nd book – Chapter I of The Spirits' Book by Allan Kardec]
Angels and demons
The beings so called angels or seraphims are not a special category with a different nature from other spirits, since they go through all the steps to achieve the highest level on the scale and be perfect in every way.
The demons, according to popular acceptance, would be beings essentially prone to evil. God is sovereignly just and good. He has not created beings inclined to evil. Demons are therefore imperfect spirits, that is to say, those who rebel against the trials and divine authority and temporarily surrender to evil.
[Taken from questions 128 to 131 in section Progress of Spirits from the 2nd book – Chapter I of The Spirits' Book by Allan Kardec]
Influence of spirits
The influence of the spirits on our thoughts and actions is far greater than one might suppose and often they are the ones who control us. Some thoughts are our own, while others are suggested.
[Taken from questions 114 to 127 of the section Intervention of Spirits in the corporeal world from the 2nd book – Chapter IX of The Spirits' Book by Allan Kardec]