Online games are social games. They make you interact with other people to make playing more fun, and to advance even further in the game. Especially in a game like WoW where you need to have a bunch of people in your party to do a raid. Because of this, you have no choice but to interact with people.
The fact that he also talks to them on the microphone shows me that he knows what he's doing. Trying to do a difficult thing on a game, while typing to someone else gives you a huge possibility that you can die on your character while typing (I have done this numerous times). So the best solution would be to talk on the microphone with everyone to make playing easier and raise your chances of not dying.
Also, just because it's a girl character, does not mean that it's actually a girl. It's sexist to say, but more guys play games than girls. Even if she were a girl, that does not mean he's suddenly cheating on you because he's talking to her. Can't he have any friends without you thinking he's cheating?
When you see him talking to 'her' outside of the game, constantly, and hiding the fact that he talked to her from you - then you can be suspicious. Right now, you have nothing to be suspicious about.
Also, yes you can be cheated on online. It's called emotional cheating. The way someone intimately talks to someone else that is not their partner, telling them things they wouldn't even tell you, and telling them they love them and stuff. That's definitely cheating, just not in a physical sense.
If you're really concerned about this though, you have to talk it out with him. Get some kind of compromise going on. Like, him not talking as often with that person anymore, or out right avoid that person.
Good luck, and remember that people online ARE real people, regardless of what your boyfriend might think.
