Hoseok is a really big quiet dude that meets Isamu initially by scaring away some dudes that were harassing him and hurting him and taking Isamu back up to his apartment to make sure he was alright. Hoseok is nonverbal like 90% of the time (the other 10% is almost entirely just people's names.) He designs and programs holographic fish for holographic aquariums, which are usually installed in wealthy people's homes or businesses as low-maintenance statement pieces. Because of this, he lives a comfortable life, and he has a lot of experience in debugging and physical hardware maintenance and computer science, things like that. He eventually ends up becoming Isamu's like…maintenance guy whenever he has an issue or just for general checkups.
Hoseok lost his jaw, most of his neck and right shoulder, lower left arm, entire right arm, and right eye in a horrifying accident when he was a teenager. He moved from his hometown to seek medical and cybernetic help and had functionally okay bionics implanted a year or so after the accident. In the decade or so since, he's slowly been upgrading his cyborg parts on the side and from much more skilled but not necessarily licensed professionals, as well as experimenting on his own parts a little bit himself.
When he meets Isamu, he finds out the receiver that connects his eye to his brain can also interpret the signals Isamu uses to interact with machinery remotely, and with a little tinkering he makes it so that Isamu can “talk” to him nonverbally by interpreting Isamu's mental voice commands into meaning. Whenever they hang out, a lot of the time neither of them say a single word (though Isamu will sometimes talk out loud on instinct, sometimes in Japanese but moreso in Korean because he thinks Hoseok would like it (he does :) ))
Hoseok and Isamu end up developing a bit of an intimate relationship as time goes on. Hoseok is just one of those people that makes everyone around them feel safe, and Isamu is no exception...so...
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