Legend gives he would never plan to be a parent, but he’s a fantastic uncle.
His sister has a baby, she’s exhausted, her husband is running the kingdom, and Legend? It’s Uncle Link to the rescue with the baby. He’s by the window, showing his niece outside when she’s awake. When she’s older, he teaches her the important stuff- how to survive in the wild, how to play different instruments, different languages, how to determine cursed objects, how to use weapons properly, and so on. Fable, the knights, maids, or the Sheikah could do that but Legend insists.
He also taught the little one how to properly set things on fire. “Drier leaves, kiddo. Yes, and don’t forget, say it with me ‘throw bombs directly at the enemy’s face.’ Mhm, good job.”
Fable loves that he’s so happy to help raise her little girl but she’s also like “Link, please don’t let her set the castle on fire.”
He's the bestest of best uncles. Honestly, considering Zelda is the one who has to run the kingdom and her husband is likely also pretty high rank and thus also probably pretty busy, who better to babysit than the queen's commoner brother who absolutely would drop everything in his personal life to take care of his niece?
Technically speaking, she has an Impa, ALL princesses have an Impa, but unlike in most eras of history, this Impa is going to essentially just be able to put her feet up because there's no way anyone can convince Legend to let someone else teach this precious little girl everything there is to know about everything.
Fable probably appoints him as the princess's godfather, mostly because he's the man she trusts most in the world other than her husband but also, Uncle doesn't;t quite cover everything he does for her baby.
Legend teaches her everything he knows, and she probably spends the summers with him so that her parents can focus during the busiest time of the year. They thought it would be good if she understood the way her people live, and she's safer than they are as long as Legend's with her, so he takes her every summer and sometimes sthey go on trips and sometimes they just stay in, but when this little lady ascends the throne, she's going to be one of the most competent and capable Queen's that Hyrule has ever seen!
Who needs tutors when you have a jack-of-all-trades for an Uncle who knows how to play at least seven instruments, speaks five or more languages, is familiar with the manners and dances of most of their ally nations, actually lived through some of ancient history and has met many of the figures from it, is a master swordsman and archer, literally talks to horses (so the best riding instructor ever), knows more about plants than the castle gardeners, can wield the same magic as her mom and has learned most of the laws of Hyrule (if only to be able to find the loopholes)? Fable just sends her daughter packing to Uncle Link and the girl comes back every time with a wealth of (occassionally dubious) knowledge and experience.
And if Fable and her husband have more than one kid? Well, Legend loves kids and he's used to the chain; he can wrangle more than one at a time.
Legend straight up is a girl dad, without kids of his own, but willing to steal his sister's if he must, and Fable is aware and happy about it.