The first adult sweater I ever made was of course, Harry Potter themed. I made it as a Christmas present for my college roommate (also a big HP fan). My Aunt Robin who is a ferocious knitter gave me a bunch of her old patterns and told me which ones would be easiest to make at my skill level. I picked a simple pull-over pattern which has you knit the front, back, and sleeves separately and then sew them all together at the end.
I love you Ron!!!
I bought Red Heart yarn in Maroon and Gold (Yeah Gryffindor!!) and plunged in. Red Heart isn't the softest yarn so you'd definitely need to wear an undershirt. Knitting a present like this is so fun. The whole time I was knitting I kept chuckling to myself thinking of the look on Katie's face when she opened the present to find a Weasley sweater with her initial on it.
I made another one the following Christmas for our other roommate Barb. Again at this time I still didn't know how to knit with 2 colors so I embroidered the letters. The sweaters turned out great. The pattern is kind of a chunky sweater which I think is perfect - looks just like something Mrs. Weasley would make!