My first IF turned real was a doll I thought my Young Adult had lost ages ago. I was a bit sad about the missing doll as I like them. Then strange messages started appearing that the top right of my screen indicating that something was running around town. My sim hadn't interacted with the doll for such a long time they were not friends. Eventually the doll visited, in doll form, and only that sim could see him.
My Sim started working on the chemistry table and after he learned all of the other potions he finally got the potion to turn the IF real. Then he had to improve the relationship and that took forever too before the IF would drink the potion.
Since then that IF has been married twice - his fist wife left him for an elder - and raised his son from his first marriage on his own. They used to be a commonly seen pair around town whenever the IF wasn't working at his ghost hunter career. He got to the top of that all while I was playing other households.
Then the game paired him up with a singe female Sim I'd added to the game and it seems to be happily ever after for them. Two of his four children inherited the IF trait and can take on doll form.