The difference between Accessory and Ally

Accessory as an adjective:

Having a secondary, supplementary or subordinate function by accompanying as a subordinate; aiding in a secondary way; being additional; being connected as an incident or subordinate to a principal; contributing or being contributory. Said of people and things, and, when of people, usually in a bad sense

Accessory as an adjective (legal):

Assisting a crime without actually participating in committing the crime itself.

Accessory as an adjective:

Present in a minor amount, and not essential.

Accessory as a noun:

Something that belongs to part of another main thing; something additional and subordinate, an attachment.

Accessory as a noun (fashion):

An article that completes one's basic outfit, such as a scarf or gloves.

Accessory as a noun (legal):

A person who is not present at a crime, but contributes to it as an assistant or instigator.

Accessory as a noun (art):

Something in a work of art without being indispensably necessary, for example solely ornamental parts.

Ally as a verb (transitive):

To unite, or form a connection between, as between families by marriage, or between princes and states by treaty, league, or confederacy

Ally as a verb (transitive):

To connect or form a relation between by similitude, resemblance, friendship, or love.

Ally as a noun:

One united to another by treaty or league; — usually applied to sovereigns or states; a confederate.

Ally as a noun:

Anything associated with another as a helper; an auxiliary.

Ally as a noun:

Anything akin to something else by structure, etc.

Ally as a noun (taxonomy):

A closely related species, usually within the same family.

Ally as a noun (obsolete):

A relative; a kinsman.

Ally as a noun:

(a glass marble or taw)