Grish is an ogre shaman west of Yanille and Jiggig, who you speak to begin the quest Zogre Flesh Eaters. After this, he really has no use, but you can still talk to him and ask him questions about the ogres in Gu'Tanoth and about Rantz, who is involved in and who you speak to begin Big Chompy Bird Hunting.
If you try to start a quest with him, but do not have the necessary requirements, you will be locked in an epic and nearly unending "No I'm not!" "Yes you are!" battle. Eventually Grish will say "Yes you are, and that's final."
You must also talk to Grish in order to wield the Ogre Composite Bow, however if you have not completed the prerequisite quest Zogre Flesh Eaters and it's own prerequisites then you will be unable to speak about the Ogre Composite Bow. If you attempt to wield the bow before speaking with him, you will get a message saying, "This bow looks quite powerful. You should talk to Grish to find out more about it before trying to use it."