Shukaido was the doctor on the moon base. He was the shy and quiet type.
He was also the one who came up with the vaccine for the virus. But by then only he, Mokuren, and Shion were alive. Since he had already been infected by the virus, he gave the vaccine to Mokuren and to Shion... Or so he says...
In reality, Shukaido never gave the vaccine to Mokuren, but only to Shion. And shortly after he died, Mokuren became infected and soon died... leaving Shion all by himself for 9 long years.
The reason why Shukaido did that was because it was to ensure that Shion wouldn't be able to follow him to next life, at least... not for a long time. It was also because Shion had Mokuren and he didn't. He knew Shion wouldn't commit sucide or else he couldn't be reincarnated.
What he did was horrible, but I think he felt guily about it... yet he didn't regret it... or did he?