I don't know much about him since I never really was interested in learning (sorry), but I do know that he's like the father figure in the group, he always gets a kick out of Hyunseung's random dancing and silliness, appears to be more on the manly side, but is actually a sensitive type of guy. Also, he also seems to have a little of a temper, or maybe that was the other members joking around and just saying that...who knows.
I do like the covers he did with Dongwoon and Junghyung, "When the Door Closes" and "I Am a Man". Doojoon does have a nice voice ^.^
Here are the two videos I mentioned above: