Speak for yourself...

Er...wha...? Not everything the party (or single character) meets is automatically an opponent, and it's also perfectly sound strategy to go and recruit an NPC to fill a hole in the party's composition:
"OK, you're a magic user and I'm a locks-and-infiltration specialist...crap, we need some muscle! And some healing! Let's ask around town and see if anyone like that wants to join us on our quest." Which means the DM can (and in this case probably should) lob at least one NPC into the party to cover one or both of those bases...and from that point on, the DM has a reason to co-operate (or not) within the party.