(Adj.) Relentless means very persistent or insistent about something ; strict or harsh -> Ex: She was relentless about how you should break up with your boyfriend. -> Ex: That teacher was relentless to that boy to wouldn't stop interrupting the class. (Adj.) Ruthless means that you have no compassion, pity, or mercy. -> Ex: Some people can be ruthless(cruel) to animals. ->Ex: Those politions are cold-blooded.