Wait , where is it ever said Sephiroth's actions are controlled by Jenova's will?
I have a... very hard time... thinking that Jenova does all that manipulation, memory mimicking, and memory eating without knowing what she is doing. She's portrayed at being too good at it to have not had a lot of insight and practice at it... she's like... a Mindflayer from D&D that managed to get a body that works like the Thing's body.
I think it knows what it's doing too, because it clearly is engaged in a form of psychological manipulation. Comparing it to a suped up Mind Flayer is pretty accurate. It posseses strong psychic abilities to read minds so it must have
some sort of mind to be able to read other's thoughts, emotions and then mimic them effectively to play upon those weaknesses and strike.
The only thing, is that while it has intelligence and wicked cunning, it doesn't seem to possess ambition or discernable ego or personality akin to a truly sapient creature. It seemingly lives just to kill, propagate and travel the stars without exhibiting any other traits that reveal its own identity. I personally do think it possesses some sapience and personality, however it's ambiguous within the text.
I can't think of anything Jenova has done which can be regarded as clear and uncontestable evidence of intelligence. SE can tell me it's intelligent, but they haven't shown me that it is intelligent. Although, again, it depends what one means by intelligence.
Right, and this is because the past exploits of Jenova are only anecdotal and historic. Narratively speaking, we are shown explicitly who the real and fully formed villain of FFVII is. It's Sephiroth. Jenova isn't dumb, but in the end it is the narrative tool, power and ally Sephiroth has at his side that enables his role of "villain." It's what set him on his path to power. It isn't the one pulling his strings or even a "character." It's as much a character as Schizo is.
Jenova is a fascinating creature given it's context and lore importance to FFVII's setting, however I feel like people get too absorbed by it and give it
far more weight than it deserves. Jenova isn't a "character" so much as it is a "monster." An extremely powerful, capable and dangerous monster. But it certainly isn't the main villain, puppet master or antagonist of FFVII.
It's like propping up the Void, the Warring Triad or Yu Yevon as the true masterminds of their respective stories. They're just lore significant tools/entities/powers that enable the villains to reach greater heights.