Generally in Final Fantasy the appearance of the final bosses have some sort of relevance to one another and/or the plot ie.
Final Fantasy VIII
1st - Ultimecia Fights
2nd - Ultimecia Summons Griever
3rd - Ultimecia junctions Griever (similar to what Adel does with Rinoa)
4th - Ultimecia begins absorbing all of reality (time, space, existence, stars and planets from across the space/time continuum)
Final Fantasy XIII
1st - Barthandelus fights the l'Cie to try and force their hand
2nd - Menrva enter into Orphan's Pool and gives birth to it, Menrva and Barthandelus are assimilated forming Orphan's protective outer shell
3rd - Orphan emerges exposed from the protective outershell
But with Final Fantasy VII i'm not quite sure of the connection between the Sephiroth who the party sees floating (after defeating JENOVA:SYNTHESIS), the 2 non-human forms the party fights then the human form Cloud fights in the end.
Above is the order they appear in. the first one (human, wearing his black coat) is what he looked like before the battle against the 2nd final boss start. the second image is Bizarro∙Sephiroth who is the second final boss. the third image is Safer∙Sephiroth who is the third final boss and the forth image is what Sephiroth looked like in the cinematic battle (where while it shows a battle it's rigged for him to loose with Cloud gaining Omnislash and if he fails to use it and Sephiroth attacks Cloud counters killing him)
My question is how does Sephiroth go from looking human at first to looking like Bizarro and Safer Sephiroth before going back to human? is Bizarro and Safer Sephiroth just more Sephiroth Clones and only the last topless Sephiroth is the real Sephiroth?