Related Rule
Germany
Practice Relating to Rule 108. Mercenaries
Germany’s Military Manual (1992) defines a mercenary as “any person who is motivated to take a direct part in the hostilities by the desire for private gain without being a national or a member of the armed forces of a party to the conflict (Art. 47 [of the 1977 Additional Protocol I]). In addition, the provisions of the 1989 Mercenary Convention apply.” 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten Konflikten – Handbuch, August 1992, § 303.
Germany’s Military Manual (1992) provides:
Mercenaries shall be regarded as unlawful combatants [i.e.] persons who take a direct part in the hostilities without being entitled to do so and have to face penal consequences. They do not have the right to the status of a prisoner of war. [They] do, however, have a legitimate claim to certain fundamental guarantees (Art. 75 [of the 1977 Additional Protocol I]), including the right to humane treatment and a regular judicial procedure. 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten Konflikten – Handbuch, August 1992, §§ 302–303.
Germany’s Soldiers’ Manual (2006) states: “Only combatants are entitled to take part in combat operations and cannot be punished for doing so. In contrast, other persons, e.g. mercenaries, can be punished.” 
Germany, Druckschrift Einsatz Nr. 03, Humanitäres Völkerrecht in bewaffneten Konflikten – Grundsätze, Erarbeitet nach ZDv 15/2, Humanitäres Völkerrecht in bewaffneten Konflikten – Handbuch, DSK SF009320187, Bundesministerium der Verteidigung, R II 3, August 2006, p. 3.