Jiren is a man who devotes everything to justice and never acts for selfish reasons; he lives to protect Universe 11, even stating that he would leave the tournament while it was happening in order to return there if any problem arose in the 48 minutes of the tournament.
When Toppo tells Jiren to participate in the Tournament of Power, he initially refuses to enter - as while it is his duty to protect Universe 11, he will not cause the destruction of other universes to save his own. He will never kill any villains he defeats and instead captures them. Even more, he shows disgust towards those who actively kill and do not follow justice. This is seen when Jiren mocks Hit's "honor" as an assassin, deeming it as rubbish. However, Jiren has a wish that he wants to fulfill, to the point that he will go against his own principles to get that wish - and as such he enters the tournament to obtain the Super Dragon Balls, although he still states that if any trouble should occur for Universe 11, he would immediately leave to help.