Grim, implacable, relentless, unrelenting, merciless are comparable when they mean so inexorable or obdurate as to repel or bar any effort to move one from one’s purpose or course.
Grim (see also GHASTLY ) usually implies tenacity of purpose and stern determination which show themselves outwardly in a forbidding aspect or in a formidable appearance; the term is applicable not only to persons or their words, acts, and looks but to things which reflect or reveal the grimness of persons.
Implacable implies the impossibility of placating, pacifying, or appeasing and is used in reference to men or to higher beings. However, the term is increasingly used to imply an inflexibly uncompromising character or an incapacity for yielding or making concessions, and, in this sense, it is applicable not only to persons but to things.
Relentless and unrelenting differ mainly in that the former suggests a character and the latter a mood governing action; both imply an absence of pity or of any feeling that would cause one to relent and to restrain through compassion the fury or violence of one’s rage, hatred, hostility, or vengeance.
Both terms often carry so strong an implication of indefinite duration or of unremitting activity that they are frequently used to describe something which promises not the slightest abatement in severity, violence, or intensity as long as life or strength lasts.
Merciless differs from relentless and unrelenting mainly in stressing an innate capacity for inflicting cruelty without qualms or an unparalleled fierceness or savagery; otherwise it carries much the same implications.