What is the submediant of e major?

The submediant of E major is C♯ minor. In the context of a major scale, the submediant is the sixth degree, and in the E major scale (E, F♯, G♯, A, B, C♯, D♯), the sixth note is C♯. Thus, C♯ minor serves as the submediant chord in the key of E major.