I'll be honest, I read too much isekai to remember so I've forgotten why he's so obsessed with our protag. Was it some ego thing about being the most supportive person to the Hero? And he can't stand the fact that she loves her brother?
I think some commenter in another chapter said it's part of his Sage compulsions or something, but that didn't make much sense to me.
Yeah, with a monthly manga, it can be hard to remember, in this world (litrpg, not isekai), jobs/protections are not just a skill package. They also come with a mental overlay that is a constant low level brainwashing. And they also are somewhat 'randomly' assigned by god, not logically assigned. Also, jobs can only be leveled by fighting/killing other beings with jobs. So a cook can't level cook without beating animals/monsters/others. It's a really stupid system.
Anyway, this basically makes it very difficult to not come to act like a stereotype of the job. People given a thief job would gain klepto instincts. Brawler get in more fights, Bandits come to want to rob, nobility classes desire to rule (awkward when a peasant gets a King class or a prince gets a Bandit class).
Gideon/Red is a guider. Its a job for nurturing/protecting a Hero and his/her party until they can stand on their own (it starts at a high level, but gains no advanced skills). It's why when Ares told him they didn't need him anymore, it hit him extra hard and he left right away.
Ares is a Sage, which desires to aid the Hero w/ his magic skills and wisdom. Unfortunately, Ares bound that up with his personal chosen one complex (Sage is a rare, high tier job after all), giving rise to a massive arrogance that resonates with his job, and he saw Gideon as a major obstacle in his chosen one destiny. Gideon is also way more competent [and acknowledged as such] because he put in the effort to learn so he could Guide the party successfully whereas Ares thinks because he's a Sage he can just do anything.