So what if Kiryu collapsed or showed some other sign of illness when he and Jack and Yusei and Crow were in the warehouse after Kiryu bombed Security headquarters? He thus didn't attack that one guard, but got arrested for the bombing despite protests from the other three boys. So Crow has to do everything in his power to get Kiryu out of there, up to and including giving blowjobs to guards. But by the time he finally does get to Kiryu, the latter only has a few weeks to live and is mostly incoherent. So for those few weeks, Crow has to take care of him 24/7 and make him as comfortable as possible.
I've had this idea for a while. The only problem is...how does Kiryu become a Dark Signer?? I don't want to mess with more things than I absolutely have to.