It's implied that Ryo became a digital human way back in brave tamer, because when he goes back in time the humans couldn't follow back, so they had to settle with sending their digimon. The reason Ryo didn't Bio-Merge is simple, though: He didn't know it was possible. Heck, nobody
knew it was possible (except shibumi, but he didn't know that the Tamers had pulled it off) until Takato did it. Ryo only found out that he could because he saw Rika Bio-Merge (we can tell because of his "WTF are you doing?" face), and considering that he spent over a year with Cyberdramon, I think by this point both understand each other rather well.
I wouldn't say Ryo is irresponsible, considering that he showed guilt for leaving without saying anything in the car ride back home. It's just that he couldn't keep Cyberdramon in the real world, for fear that he would attack anything in sight. In other words, he was given the choice between staying at home partnerless, or go into the completely unknown Digital World alone
, without any way of return next to a bloodthirsty
partner. So I would say that he is very detached from humanity, but you might want to argue that he is completely insane.
Thematically speaking, it makes sense for Arkadimon to be the only one without to have a proper name, considering that Neo specifically raised him to be a weapon.
edited 2nd Oct '12 1:17:18 PM by luislucas