A co-defendant is a person that is charged alongside another person or persons for the same crime or civil matter.
In order to be a co-defendant, the person must be charged with the exact same thing as the first defendant. There may be scope to limit their charges based on who bore more responsibility for the alleged legal issue.
A co-defendant is allowed to have their own legal counsel and have the same legal rights as a person that is solely charged.
A co-defendant can be found not guilty even if the other defendant is found guilty. Although the cases are held at the same time, they are essentially two separate cases as it relates to each defendant. Each defendant makes their own defense and plea. If different legal teams are hired, all information is shared between them for the good of both defendants.