His mother, though she can have children, just wanted a very large family. So she turns to adopting some as well as having her own. 

Ignis was only a baby when they find him, and really, his family’s features are so diverse that he could have been related to them anyways. He is also the last child they have/adopt. SO HE’S THE YOUNGEST. 

It’s not until he’s 5 that his parents sit him down and explain that he’s adopted. But not to worry, because, surprise, the Scientias actually known for adopting people into their family rather frequently. 

He’s not the only one who was adopted they tell him. There were a few cousins, one of his siblings, a few aunts and uncles, and somewhere there’s a grandmother who had her adoption annulled a few times due to filing errors and was finally adopted correctly the fourth time. (Honestly.)

But they reassure him that they love him just the same as everyone else, but they felt it wouldn’t be fair to him that he didn’t know. Just in case he wanted to find his biological parents one day. 

So he grows up knowing, and honestly, there is no difference between how they treat him and his other siblings. (He has 6.)

Though…there are some issues when his parents have to choose one of their children to work under his uncle in the capital city of Insomnia. 

None of his other siblings express interest in the position but him. So, at 7 years old, they let him go. And the king welcomes him as Noctis’ friend and future adviser with open arms. Not to mention his uncle, who was currently one of the king’s adviser, dotes on him more than he should.  

But the nobles of the capital were…less then pleased that they sent one of their adopted children instead of a bloodline child. Indeed, some viewed it as a slight.

…or did until his mother came all the way from Tenebrae after his first letter home has traces sadness in it’s words. And it’s been a long time since one of the nobles of the Scientia household grew truly upset with the other noble houses, and it nearly starts a feud. 

She reminds them that the Scientias, though they serve the king, are one of the highest members of the noble houses, and that they dare question the status of one of HER children speaks to how little they must regard the king’s opinions. (It nearly comes to a duel before the King realizes that the Terror that is Lady Scientia has returned to the Citadel, and comes to personally stop the arguing.) (He also really REALLY wanted to see one of his old childhood friends again.)

That settles that. Ignis is officially the future adviser to Noctis. 

(Also, his mother then makes it a point to plan a yearly family trip to Insomnia just pop up and frighten the other nobles for a month at a time.)((And it’s never at the same time of year.)) 

The whole situation does make him wonder just where he did come from. So he waits until he is 18 and has some free time to locate his birth family. And when he finds them, they’re very glad to see he found a future, and they part on good terms. He doesn’t go back to see them after the first time, not wanting to disrupt their lives again. They’re happy, and so is he. There’s no reason to keep kicking up dust. 

Also, Noctis doesn’t realize Ignis is adopted. At all. No one tells him and it slips Ignis’ mind until Noctis is 16 and meets his WHOLE family at once. 

“You all really don’t look alike.” Ignis would snort upon realizing he never told him, and just…not tell him for the rest of the night. 

Noctis figures it out when Ignis’ siblings are teasing him and he says something to the effects of “At least mother got to pick me. She was stuck with you.” (Jokes along those lines are as natural as breathing to this family.) To which one of his older siblings would be like, “She picked me first.” 

(But then he realizes that Ignis is the BABY of the family and never lets him live it down.)