The cha'DIch is a position within the Klingon legal system and is essentially a warrior's 'second' during a trial. This is because during the trial, the accused is denied all combat privileges, so a cha'DIch must fight for him when required.
In 2366, Kurn served as Worf's cha'DIch when he challenged the Klingon High Council. When Kurn was later injured during an assassination attempt by Duras, Jean-Luc Picard took over the role. (TNG episode: "Sins of the Father"; TNG eBook: A Sea of Troubles)
By 2380, Federation Starfleet Captain William T. Riker had developed a close to fellow captain Carol Freeman that Riker considered her to be his cha'DIch, much to her annoyance. (LD episode: "No Small Parks", LD Logs episode: "Personal Log-Carol Freeman"