I think the relationship between sakuraba and shuuji is more like, dependant? Sort of? It's hard to explain. From what I understood from shuuji's character is that he was born with everything. He supposedly had the perfect life. And I think because of this he became miserable. Life was easy for him, no problems and such. And thus that became a problem. I think that's also why he became a womanizer and kept making the people around him miserable. He wanted to make his life more interesting. Because he himself was miserable, too miserable. Too miserable to care about others. Except for one. Our mc. Well not "care" as in for his well being but like care as in he doesn't want him to leave his grasp or at the very least not die. And I think shuuji knew that he was an unforgivable bastard too. I'm not too sure about they're relationship of wether shuuji views him as a toy but one he doesn't one to break or maybe he felt that comfortable around him despite ruining his life. But that's just my understanding of shuuji's character and his relationship with sakuraba.
But hey, at least now we know who's the father! Definitely not shuuji tho.