Aeris is taking advantage of a possessed/infected amnesiac because she's on the rebound from the guy she thinks ditched her for an older lady. She's suffering from wounded pride, heart break, and a loss of confidence in her capacity to judge others.
Ironically, the fact that she thinks she misjudged Zack compelled her to become a *better* judge of character, which is why she's able to figure Cloud out more quickly than anyone else. It also helps her professionally because living in Midgar, a young lady is an easy target. Trust issues combined with her unique abilities resulted in her leading a lonely life.
To her credit, she comes clean about what she wanted from Cloud and is intent on not using him anymore, in the interest of what's best for him instead of what she wants for herself. It took a really long time for that to come to pass, and she never had the opportunity to properly meet the "real" Cloud to see if there was actually anything there for them.
Now, considering the other side of the coin, Tifa, she wants alot of things from Cloud. She wants a strong ally to help take down Shinra. She wants someone to restore what was taken from her in Nibelheim. She wants someone to help her to overcome her chronic feelings of loneliness and abandonment.
And then out of nowhere Cloud comes back into her life like an answer to all her prayers. He's too good to be true, but at the same time far from perfect. She has powerful feelings of nostalgia, which blind her, hopes and dreams that had been all but snuffed out. To top it off, she also has gaps in her memories. No wonder she's so hesitant and confused.
It takes a very long time for Tifa to fully admit to her own failings as far as Cloud is concerned, at first because he treats her with a degree of indifference that could almost be considered unfriendly (when it might be just the opposite in reality), and then later when he seriously starts to unravel mentally and he needs all the support he can get. She's completely useless when he finally collapses, all because she has to deal with the fact that the Cloud she knew means so much more to her than the person before her that doubts as to who he is conflict mightily with what should be done to help him.
At any rate, Aeris was personally in a much easier situation than Tifa, and her demands of Cloud were also much simpler. If Cloud actually had a mind of his own, the easy choice would have been Aeris.
I could do a write up of Kikyou if you want. Her situation is weirder though.
Ain't there a wiki already?
Although you can't say Kikyou was reaaally involved in that "love triangle" avowed by the fandom, in the first place. Anyone who had seen the series/read the manga can tell you her real goal wasn't winning Inuyasha over.
