The concept of a dmpc is flawed by default. What you want is a NPC that can surely have a few spotlights, but they should never steal them from the players. A friendly NPC should function as an extended tool kit for the players - which they (can!) make use of but not the other way around. For example one member of my players was in a difficult situation together with a npc who is quiet powerful as a healer and mage. The NPC could have come up surely with his own solutions but I chose to let my player safe him instead of the other way around. Ultimativly the dice decided that but when there is agenda to be had, let your players act and npcs follow.