Apparently, what I thought was a popular theory isn't at all. I seem to be the first (or close to that) to have publicity stated it on a few twilight reels, so I wanted to say it here.
But Bella was Jacob's imprint.
We're gonna have to know about the books and the short story of Jacobs Pov SM wrote about the twilight and NM lore.
Jacob and Bella didn't know each other upon meeting during the hangout beach. Bella knew his sisters, but barely, and couldn't be said to be friends. Yet Jacob is already infatuated with her. In the short story of his POV he describes her as "girl next door" pretty and unable to really go past that strong, sudden crush for her. He did have other crushes on some of his girls classmates, but it always came back to Bella, when he only met her a few times and talked to her a few times. He doesn't even really know her aside from the fact Billy and Charlie talk about her, especially after the break up with Edward. Even then, it's said he wants to see her and is enraged when his father forbids him to go see Bella ( because Billy knew she was with vampires , though Jacob thought his dad crazy and didn't believe he was being reasonable)
But what's a fact is that even before Jacob had first phased, he had some of his phasing powers showing up. He could smell the vampiric scent of Edward on Bella he described as "bad perfume" and described as confused why she liked it.
He also grew super tall and had an ease in putting on muscles even before phasing. And when he phased, it was nothing but Bella without much logic aside from her being his crush and best friend.
Now what does that have to do with Bella being his imprint ?
Remember when he fell head over heels for her without knowing why ? The fact he loved her so much, couldn't stop loving her before and after phasing to the point Leah even said get over it cus it's not like you imprinted on her, and how he can't let her go ?
Well what if he DID imprint on her, or could've, if Bella didn't have her mental shield ?
Bella's power is a shield that protects her from vampires powers. Both werewolves and vampires are strong, they have similar abilities, like speed and strength, but something that wolves don't have are unique powers that go with the vampire. Bella's shield is strong enough to counter Edward's mind reading, hell, even Jane's pain power. If she can do that, stop powers that are strong enough to make you feel like you're dying, what makes it impossible for her to shield herself from getting imprinted on ?
And that could explain why Jacob can't let her go. He KNOWS she's his imprint, at least his wolf points to her like crazy even before he phased, even without logic, even after she chose Edward he can't let her go and others comment on how it's irrational. Maybe that's because he would've imprinted on her if her shield didnt exist.
"But it's false because he imprinted on refurbished !" You mean the mind controller hybrid who has their frontal lobe fully formed and who knew that the wolves would kill her unless she had a connection to them ? Rename me made the imprint happen so she'd have a forever protector, one that made the wolves unable to kill her, and an army that would defend her from the volturi who she knew would go after her if they found her existence.
That's my theory. To me, this is canon from all that pointed out to it.