Sephiroth pretty much seems to be the most....perspective and upfront of the rogues, barring Jecht. He puts on no airs, in terms of how he works with the villains. He just keeps to himself, does what he wants, and basically is just in it for the chance to piss off Cloud and fight strong opponents.
In fact, he actually has several moments where he feigns sanity rather well, especially when talking with Warrior of Light.
Not to say he doesn't have his moments of cruelty (i.e. beating up Firion and stealing his Red Rose just to provoke Cloud) but he doesn't "act out" or go batshit insane like the others or spend his time plotting...unless it's for Cloud
It just shows how Cloud and Sephiroth have a warped bond, despite how much Cloud hates it. Sephiroth in his own twisted way enjoys fighting and confronting Cloud, despite hating his guts and trying to kill him. Despite how he looks down on Cloud and resents him, he begrudgingly respects him in his twisted way. Even though he knows now his words and actions to manipulate Cloud like a puppet don't exactly work, he truly gains a sick satisfaction in provoking Cloud's anger and predictable response while also calling him a "puppet."
...They truly do have a strange theme of "predestination" regarding their confrontations, and it's rather interesting Cloud accepts it while also rejecting it.