When you're hungry, at work, they already know your food order. When you're talking, s/he can almost always finish your sentences. S/he can read your body language and facial expressions and know exactly what you're thinking without you having to say a word. And when something goes down at work, s/he is down for you like four flat tires.  

If you're spending 40 hours or more each week at work with someone like this then this is likely your work wife/husband.  

These are the people who get you through the day, who encourage you to give your best at your job, and who make you laugh during lunchtime chats. And work spouse relationships can go much deeper than a simple break-room friendship