Based on what I'm reading, I get the impression that he wound up trusting the wrong people and falling into a bad crowd, maybe becoming loyal to them? Perhaps disowned by his parents and left homeless because of it? In general, getting into a very poor relationship with authority figures.
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What is the dynamic between Oliv and Jason? What causes them to clash, and how has it affected Oliv in the long term?
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HEY that's actually like, 90% correct! Like, totally on point about Oliv's childhood insecurities.
As for Jason, yeah, he's always been a very "kill or be killed" type, but in a way that's hidden under an approachable exterior. He's extroverted and seems generally fun to be around, but will also drop people and alienate them in a snap, even over very small things, or sometimes for no reason at all. He definitely fears commitment, whether the relationship be romantic or platonic. He will tend to use wealth and charisma to make friends and/or date, but he never lets anything progress and barely ever really shows his true colors. He's dropped/dumped some people in very nasty and scathing ways, too.
As for Oliv, well, they'd known his true nature from the get-go, as they were the person he would take it all out on, thereby causing most of their childhood insecurities. As they got into their teen years, they pretty much started cluing in on his bullshit and how much he'd kind of messed them up. That's where the clashing started, as they started to fight back. Jason takes criticism poorly, and started lashing out at Oliv in various ways, ranging from passive aggression, to locking them out of the house when their parents weren't home. He even broke some of their belongings a few times when very upset. Between Jason's retaliation and being cut off from Leo, Oliv hit an all-time low emotionally. They retaliated in return, and things would escalate to physical fights, which... really, even back then Oliv was much stronger than Jason so he'd end up far more banged up than them. Camber started to intervene seriously, and the siblings just started avoiding each other altogether. By the time Leo was with Amy, Oliv would use that as an excuse to leave the house often. As for when Leo had to move into the Velasquez house, it just got... more awkward, but they still kept avoiding.
Now having moved out, Oliv almost never sees Jason. Their only encounters as adults have been to visit their parents, and at least one of these instances led the siblings to get very drunk just to try and put up with each other, which actually just made things ten times worse. So, they don't visit their parents at the same time anymore.
SO UH BASICALLY yeah, very bad, Jason pretty much emotionally abused Oliv when they were kids and overall their relationship just got worse and worse over time.