intervenor

…with defendant-intervenor's participation

An individual who is not already a party to an existing lawsuit but who makes himself or herself a party either by joining with the plaintiff or uniting with the defendant in resistance of the plain-tiff's claims.
http://legal-dictionary.thefreedictionary.com/intervenor

records to Plaintiff without Defendant-Intervenor's participation, Defendant-Intervenor will have no other opportunity to advance his interests. ...
www.mplp.org/Resources/mplpresource.2006-10-25.0026431467/

the court here affirmed the district court's dismissal because plaintiff's complaint lacked the participation of defendant-intervenor, a co-owner of the ...
www.lexisone.com/.../Federal-Courts-Federal-Circuit-Court-o...