idk what it is exactly that u dont understand so lmk if u want more info but basically this is a guideverse where there's espers who have like super powers n shi and they need guides to regulate their powers bc if they use them too mcuh they kinda go berserk. anyway mc dies in the beginning and he goes back 6 years ago in time and he like decides hell prevent disasters that happened in his old life that impacted the outcome of his life before he died, for example ml's friend dying, mentosa island and allat. hes basically just making the wrongs right in this life because we learn from him and the terrorist attack that, well, there is a terrorist organisation and he died bc he failed to defeat them. thats kinda the whole reason hes doing this.
so in short hes doing this is to defeat a really big terrorist organisation when he failed in his old life to do that, and every disaster is linked to that organisation
i have no idea how to explain jaechan's personality tho cuz i dont rlly know either but id say hes like yearning for ml and he regrets treating him like shiet in his old life (f.e trying to "bind" ml to him) just bc he loved him, hes trying to do better in this one by "distancing" himself & trying to bring ml and the girl tgt but i think he fails cuz he just harbors feelings too genuine and important for ml, i hope i explained what u ddint understand properly ^^
I think you nailed everything but Jaechan's reasoning / motivation for doing what he is doing. He couldn't care less about the terrorist org itself if the org had not tried to harm Woojin. Woojin and Jaechan's aunt are the two people Jaechan loves more than himself. So he does not care he died the first time around. And he also doesn't care if he dies this time around too, he just wants to resolve things that would make the life of his loved ones better even if it means taking down a huge terrorist org that almost took over their country in past life. This is why he has a devil may care attitude and jumps head first in every danger w/o care for his own life and Woojin realizes that Jaechan has no care for his own life and he almost comes close to dying multiple times. Ofc Jaechan was part of the intelligency agency and wprked in the field during the 1st TL so he does know how to assess danger and make quick decisions in the field but he does puts himself the very last on the list of things he should care about. So yeah, the whole grand plan to make Woojin's life better be it mentosa island, the current arc or any upcoming arc and then let Woojin live his life w/o interfering(which is moot since this whole plan has Jaechan interfering in Woojin's life bc Woojin couldn't care less about his 1st TL GF). The only issue, Woojin decides he wants Jaechan to be a part of his life so Jaechan has to switch plans later on.

am i stupid bcs i don't understand the story at all (●_●) like the story's actually really compelling but i can't understand the details of their world and mc's personality bcs he's contradicting