As a plaintiff, it is a good rule of thumb to expect a defendant to file a counter-lawsuit. Strategically, defendants employ counterclaims to put a plaintiff on the defensive and create risk for the plaintiff. Substantively, a counterclaim sometimes must be asserted or else it is lost/waived. Rule 55.32(a) requires a defendant to assert, as a compulsory counterclaim, anything which arises out of the same transaction or occurrence. If you don’t use it, you lose it. First Community Credit Union v. Levison, 395 S.W.3d 571, 579 (Mo. Ct. App. 2013).
A permissive counterclaim is broader. It allows a defendant to allege “any claim the party has against the opposing party which is unelated to the opposing party’s claim.” Id. Therefore, it is possible to encounter a permissive counterclaim which is totally unrelated from the reason for the lawsuit being initially filed.