A companion is one who serves as a friend or partner in something. Whether it’s travel or dinner or card-playing, your companion is the one who does it with you.
The word companion is a close, um, companion to the word company, and you might as well say that someone you keep company with is your companion. Sometimes, it describes a piece of art or other work that goes with another. For instance, you might read a biography of Emily Dickenson as a companion to her studying her poems. The things go together and complement each other.