who is diablos?

Simply put, he is a Guardian Force in Final Fantasy VIII. To be more complicated, he is a demon in his own dimension, trapped in a magical lamp given to Squall by Headmaster Cid before their first mission as SeeD.

To get him join the party, you have to use the Magical Lamp and fight him. I would encourage you to fight him early on because he has some useful abilities like Mug (which is totally awesome to have by the time you reach Deling) and Time Magic refinement.

Here's some screenshots of Dark Messenger:


As you can see from the last screenshot, Diablos is my last hope before it's game over for me. Ha.