I'm a new player to RPGs and I'm trying to incubate my skills so that I can play in a group one day. I've got a fair grasp of the rules and I've been running one on one campaigns with my wife; both as a learning experience for myself and as a way of enticing my wife into the system as well.
I'm getting to the point whereby I feel as though my book-learning has plateaued and the only way I can become more skilled is through playing actual games. My wife wouldn't be able to GM, as I don't think she's that into it.
Is there a way to GM and play as a character that is fair to all sides and doesn't leave my PC at an unfair advantage? I'm playing in Pathfinder specifically.
I stress the word fair in this instance as I want to ensure the rules are obeyed and that nobody is treated differently.
This is somewhat similar to the question How can I GM and play a PC without spoiling the fun? however, this is equally about use of this method to help with educating myself and my wife about how the system works from both a combat and dungeoneering setting.
DMPCs do often tend to steal the spotlight, but I've seen it work well under a few circumstances. Basically, if you don't want to give your PC an unfair advantage, do the opposite. Give them unfair disadvantages. Trust me, no one will complain about that.
A prominent mapmaker needs to make a map of some extremely dangerous territory. She hires the PCs to protect herself. She's important to get the PCs paid regularly, and absolutely must come along with the PCs in order to actually map the territory, but she's basically useless in every other regard. She spent almost every combat hiding behind the tanks. Occasionally she would cheer them on (a la bard), but usually she would just hide.
Why this worked
The mapmaker didn't challenge any of the PCs at what they were good at. She let them excel, even demanded it of them. Where the PCs were good, she was useless. People like to be good at things - let them.
However, she wasn't an idiot. She was a real person with thoughts and feelings. She didn't annoy the PCs by constantly making stupid decisions. She was a silly person with silly concerns that the PCs didn't care about, but she didn't constantly throw herself into danger. She helped the group when she could, and stayed back when she knew she was out of her depth. You don't want your RPG to turn into a game of "protect the idiot".
The One-Trick Pony
A group of pirates (the PCs) run across a shipwrecked man and woman. The woman turns out to be a cleric of a god of prophecy, and she foretells the end of the world while recovering from her shipwreck. After some argument, it is decided that ticking off the god of prophecy would be a bad idea, so the PCs go to save the world, even if they are convinced they're gonna die in the process. The prophet (and her bodyguard) come along, because it was her stupid idea in the first place.
The prophet was a useful cleric in a party that didn't have one and played a primarily supporting role, healing PCs that need it. She was clearly there to Advance the Plot, but the things she prophecies usually aren't helpful, but consisted of "Stop this volcano from exploding" or perhaps cryptic directions ("Find the Avatar of Hope") that only become clear after they've discovered the thing anyway. She had secrets, but nothing she knew could help them save the world or defeat the boss.
Moreover, she was a real person and many of the PCs became close friends with her. As the game developed, she went steadily insane (in-game side-effect of seeing the future). While many players would have cast her aside without a second thought, because they were friends the PCs continued to bring her along and care for her, insanity and all, and even tried to save her in addition to saving the world.
Why this worked
None of the other PCs were clerics, so she was important to the survival of the party. She had to be there, but she didn't do anything but healing. Well, and giving them generally unhelpful prophetic messages of doom.
She actively befriended the PCs, and ended up telling them her secret as a mark of trust. It did nothing to change the plot, but it built a bond between her and several of the PCs. They liked her, and wanted her to be around. (One of them even fell in love with her, insanity and all. It was adorable.)
What didn't work
Her bodyguard actually ended up being too powerful, so I would have killed him off early if I were to run it again. His goal of "protect the prophet" generally translated to "kill whatever's threatening the party" and the party didn't really need another tank. They could have protected the prophet just fine on their own.
To Sum Up
Keep in mind these two goals: Need and Friendship. There has to be a mutual need for the DMPC and the party to stay together, and they have to end up as friends, not resenting each other.
If your DMPC is too powerful, doesn't have a specific need to be there, or antagonizes any of the PCs, kill them off. You don't need a DMPC to have fun.