I. P. Grimly is the major and sarge of the war. Doesn't depend on what team you have, I. P. Grimly is always your major. So it's very unclear to say what nation he really is, but from the cutscenes, he seems to be from Tommy's Trotters as of his clothing, but from Piggystroika as of his name.
His name was inspired from Russian psychologist Ivan Petrovich Pavlov, commonly I. P. Pavlov. Grimly's first two names are unknown, but might be the same as Pavlov's.