I like buddies that make me feel comfortable in and out of the water. Buddies make diving safe and more fun.
Being a good buddy starts before the dive - buddy checks, helping with gear, verifying the dive plan and objective, agreeing on signals amoungst other things, hands expensive stuff like cameras to you so you don't have to jump in with them.
Good buddies during the dive are always close enought to help if there is a problem (or better yet before the problem occurs), usually within arms length, checks both air guages, looks for neat things and points them out to thier buddy, stops you from doing dangerous things, and lots of eye contact to make sure that you are OK. ( I also like to fool around with my buddy during the safety stops
)
Really good dive buddies stick together after the dive - sign log books, verify depths and stuff, checks my PG calculations, helps put away gear, swaps neat stories about the dives, hopefully shares the pictures , and swaps phone numbers so you can do it again.
There is a lot of help other than the stuff that I said above too.
But remember - it is nice to have a good buddy, it is nicer to be a good buddy and it is nicest of all to be a part of a good buddy team.