The one the spell is cast on gets half defence, but the caster himself also gets defence halved since he cast a spell. The caster, however, can have normal defence again if he defends after casting.

Disbalance should be an offensive spell (Solstice had it). If Solstice casts Disbalance on Faw, for example, Faw is the one who has 3HP removed but both get their defence halved.

EDIT: Can we go Not Dennis?