I believe that,in some strange sense, Sephiroth considers Denzel as his child, because to sephiroth everything Cloud's is his too. So if Cloud sees Denzel as a son that make him sephiroth's son too (that's why the remnants bring Denzel to their children army,they know his "father" saw him as his child)
Technically the reason they bring Denzel with them to the Forgotten City and turn him into a puppet of sorts doesn't have anything to do with Cloud. Denzel had Geostigma, and Yazoo was gathering up all the children with Geostigma.
But this idea has really solid legs, and I think we could apply it to Marlene instead. Cloud doesn't really see Marlene as his daughter, but I feel like he definitely views her as some sort of kin. Marlene clearly views Cloud as someone important too, likely as a sort of kin member as well. I wouldn't call him a step-dad, but like, a really involved uncle.
And Marlene doesn't have Geostigma, which makes Loz's choice (or perhaps Kadaj's, since it seems that Kadaj calls Loz to tell him off and remind him to kidnap Marelne) to take Marlene motivated by the desire to draw Cloud out to them.
ALSO. Years ago I read a sefikura fic called "Raising Denzel" where Sephiroth basically took in Denzel after Cloud started avoiding his friends and he taught Denzel to fight and manipulated him into believing that Cloud needed to be protected by them. Sadly although I recall the name (I had it bookmarked on my old phone), it seems like the link I had saved is no longer good. I'm not sure if it's lost to time, but it anyone knows where it can be found, PLEASE SHARE! I remember it being super good!!!
