Missouri can be considered half-south, both
culturally, and in terms of its alliance in the civil war. They chose not to secede from the union, but they also chose (and
convinced congress) to admit them into the union as a slave labor state. They rejected slavery during the war later, but were, to some degree, waffling about the related issues of slaves and secession.
Whether missouri, as a state, is southern or not depends on who you ask, or which part of the state you visit. Much of the southern end is culturally southern, while the northern end is largely midwestern.
A few other states we similarly divided on the issues -
http://en.wikipedia.org/wiki/Border_states_(American_Civil_War)