Wis. Stat. § 799.43: Defendant’s pleading in eviction actions.

The defendant may plead to the complaint orally or in writing, except that if the plaintiff’s title is put in issue by the defendant, the answer shall be in writing and subscribed in the same manner as the complaint. Within the limitation of s. 799.02 the defendant may counterclaim provided that in construing s. 799.02 as applied to eviction actions, any claim related to the rented property shall be considered as arising out of the transaction or occurrence which is the subject matter of the plaintiff’s claim.