At MoonSpiritBoutique, we largely use terminology associated with the "Spirit Keeping" branch of spirit work, rather than "Spirit Companionship". This is due to how we first started out, and also to avoid confusion when speaking about the human and their beings.
Spirit Keeper: The equivalent of "Spirit Companion". This is a human that has taken entities and/or spirits under their care with the use of vessels.
Spirit Companion: Used when referring to the companion (entity/spirit) of a Spirit Keeper.
Entity: Refers to any being that is living, and does not classify itself as a Living Organism within Scientific Terms. (Example. A dolphin (living or dead) is not an entity. But, a living unicorn is an entity because it is not documented within biology as an organism living on this planet.)
Spirit: Any entity that is now dead. All spirits are considered "Intelligent" by MSB's terminology, and do not age, procreate, or learn new skills.
Ghost: A spirit that is unintelligent. A ghost is trapped within their memories before death, and are unable to interact with others outside of the context of their memories.
Binding: Work that is done by a human to enhance communication and energetic manifestations by companions.
Attachment: Work that is done by the spirit/entity to anchor themselves to a physical object.
Vessel: The physical object that holds a binding (Pendant, Ring, Statue, Doll, Etc)
We use the current "White-Grey-Black" Arts Format when classifying our companions based on moral code.
White Arts: These beings will never harm creature that are innocent, and they do what they can to preserve the happiness and health of the group rather than the individual. If one person's sacrifice means ten people will survive, then they will take that route. They always tell the truth, and usually follow some kind of higher authority (Example: Angels).
Grey Arts: Beings classified as "Grey" have many similarities to White Arts. They do their best to not harm innocent creatures, and they work for the group as a whole rather than the individual. However, they respect freedom from authority as long as it does not lead to violence/chaos.
Dark Arts: Companions within the Dark Arts category see the needs of the individual more important than the group, but will not go out of their way to harm others to get their own happiness. They desire freedom and do not usually follow an authorititative figure, though they may still have Kings/Queens/Rulers within their society to aid in the overall group.
Black Arts: Black Arts Companions put the needs of themselves the highest. They will not aid others, unless there is a mutual benefit. They do not follow authority and instead will rebel against control of any kind.
No "Arts" is more good or evil than the other. It all depends on perspective and what you value. If you value freedom from authority and being in control of yourself, Black Arts companions may be best. If you prefer structure and aiding the group no matter the cost to the individual, then White Arts would be a good choice.