Who owns Copyright?
In principle, copyright belongs to the creator of a work. But if the work was created in employment (payroll), then the right often lies with the employer. If a work was commissioned, then in exceptional cases the copyright may lie with the client.
If it was created in collaboration, then both creators have joint copyright to the entire work. If the work is separate (for example, a writer and illustrator create a book together), then the copyright on the individual contributions lies with the person who created that part of the work; the illustrator has the right to the pictures, the writer the right to the text.
Incidentally, other agreements on the division of rights can be made in writing.