>>280641367
Technically he didn't even join Border to protect Chika. He was just angry at his own powerlessness. For that matter, we don't know exactly how he joined Border. We just know that he applied, didn't get accepted, was told it was due to 'low aptitude for trigger usage' while those who were accepted were getting their paperwork from the staff, and then met Jin after breaking into the emergence zone.

So the not particularly social Osamu had no particular goal beyond being able to do *something*, joined Border in a non-standard way that he missed a chance to socialize and connect with those accepted during the same time period, possibly missed out on a provisional enlistment and the training and explanations that would include, and embarrassed himself on the trion soldier test after the formal enlistment ceremony.

Leaving him starting his career at Border with no clear goal, no time pressure, no connections, less training and information than some of his peers, and the only stand out thing he did was fail miserably.