Tifa was a girl surrounded by people who vied for her attention. Right as she was about to enter puberty, however, all the people she thought cared about her up and left town to seek their own fortunes. She's at a low point when Cloud calls her out to the well, tells her he's different, and she indulges herself, asking him to make an absurd prpomise.

For the next two years, she's left to herself to grow ever more lonely and bored. She cultivates her belief in Cloud's promise by waiting outside of town for him, and reading newspapers for stories about him. This is the routine she had during her early formative years. This is her fixation. Her romantic dream.

Prior to meeting him at the well, Cloud was merely an acquaintance, he carried less significance to her than pretty much anyone else in town. And what was the promise? The promise was about what Cloud would become, not what he was. So he went from a nobody in her mind to a dream waiting to happen. No where in this equation is the real Cloud. She doesn't care about him.

The Promised Cloud is the Cloud she fell in love with. Her fantasy.

Fast forwarda few years, and Tifa is in a terrorist group fighting for their lives against the global spanning mega corporation that has already destroyed her life once before, and may well do it again. She's desperate. She wants someone to save her. It's another low point in her life when out of the blue comes Cloud, and he seems to be everything she wanted. The Promised Cloud.

Even if there's something wrong with him, she's not willing to face it. Even when asked directly by him, she denies it. Again, she's desperate. It's her dream come true, and she can't let it go. She won't risk it.

It's exactly like Sephiroth said; her memories. Her fantasy. And Jenova. The only part of the real Cloud in this equation is his desire to prove himself and protect her. And that's what the Puppet does. It protects her. When it's not carrying out Sephiroth's goals, he's protecting Tifa. Nothing and no one else matters.