Thanks to Pawn315 for the inspiration!

TL;DR opener (it really needs it): FF7 – Hojo was forced to raise Sephiroth as his “adoptive” father/legal guardian.

Regarding OP, I actually like to think that Hojo, after giving his biological son to science was then forced to also be his caretaker/legal guardian. Gast, being the head scientist, would be busy with other projects and such. Because of this, he left the taking care of the experimental Jenova-child to one of his aides, Hojo. It would be important that somebody working on the project was in charge of the child so they would have proper knowledge and context for dealing with and noting down information in response to various occurrences. Hojo, in turn, would develop strong thoughts on the subject since he is constantly around it. This would give rise to his Jenova Reunion theory. Hojo, with his personality, would not be a good guardian which would explain Sephiroth’s dislike of him generally (he claims Hojo is not a good scientist but Hojo’s theory is validated and very little is shown to us to indicate that Hojo is anything less than Gast) and would explain Sephiroth’s laughter about his “father.” Sephiroth’s views about him all stem from their personal relationship. In contrast, Sephiroth seems to respect Gast. Further, Hojo indicates that Sephiroth was not aware that Hojo was his biological father (during the assault on the Mako cannon) so again it would make sense that Sephiroth’s laughter about his father would be about an “adoptive” father or father by circumstance. He respects Gast so would not likely laugh about him. Not respecting Hojo, he would laugh.

But…this is true, right? At least, that’s what I always thought.

What do you all think?