A good running buddy shows up, and shows up on time.

That's the most important thing. It's my main reason for running with buddies: 'cause I know they will be waiting for me if I don't get out there. And they come out because they know I'll be waiting for them if they don't get out there, too.

Now, one thing I find with some people is that they take off way too fast. I think I tend to do that when running with an unusual running buddy, too. I find it important to voice things, not to be afraid to ask to slow down if necessary, or "would you be okay if we ran a wee bit faster?" or "is it okay if we walk for a minute every 10 minutes?" or whatever.

I agree with Yellow that it's okay to change your running plans a bit to accommodate a partner once in a while. It's a great motivation to have someone out there waiting for you.