And speaking of his siblings, who do you think we will have the absolute worst first impression with?
Assuming all other Primarchs stay the same until our arrival.
Lion El'Jonson: -Their respective times on their planets was quite similar in that they both fought against monsters and abominations. The big difference are their respective father figures. Morty killed his father, while Lion promoted his to second in command. It could lead to friction, because that is definitely something the Lion would find incomprehensible at first. The farming is tricky. Caliban is a feudal society, so Morty actively bevoming a farmer might lead to the Lion looking down on him. Feudal society, yeah? Appreciative at best, but probably marked with tensions and shows of superiority.
Unknown
Fulgrim: Planets are very similar so they might share a common origin and both strived to improve the lot of their people in their own ways. It's just that their personalities might clash tremendously. Or they might get along? Not too sure. It's either they hate each other or they will be good pals, I think.
Perturabo: Similar attitude, but current Mortarion might not get along with Perti's extremely harsh measures. I think it will start out rough, but if they continue to interact with each other, their relationship will improve abd they will have a positive effect on each other. Best case.
Jaghatai Khan: Don't know enough, but farmers and nomads don't mix. Prejudices for the win? Armies might complement each other very well. Probably a neutral relationship.
Leman Russ: oof. Communication issues exponentially increase. Probably a bad outcome.
Rogal Dorn: neutral, probably.
Konrad Curze: Might actually hate him.
Sanguinius: I think they could get along, but Sanguinius might actually come off as too perfect, so Morty will definitely be wary for some time.
Ferrus Manus: Don't know enough about him.
Angron: Let's just keep our distance...
Roboute Guilliman: Difficult. I think this will be more professional. Both can appreciate their respective strengths, but they probably just don't really click?
Magnus the Red: ehhh... Suspicious until proven innocent? Or at least until proven to be more cautious.
Horus: Everyone likes Horus. Morty might dislike his showmanship and put on attitudes though. Deoends on how much effort Horus expends on us.
Lorgar: Can't say. Probably neutral.
Vulkan: Probably positive. Don't see a reason for them to truly dislike each other. Their opinions might widely diverge though. Depends on their respective humbleness and respect.
Corvus Corax: Can't say. Probably neutral.
Alpharius/Omegon: ?
Edit: always add a bit of initial disdain because farming.