Hakuba should propose to Kaito then the white imagery would fit his civilian identity too via giving him his surname. Plus Kaito keeping people in the dark and operating in the shadows (black) and Hakuba recognising the good in him and being determined to bring everything about Kid into the light (white), also the Kuroba or clover on the black hat accessory reference in Kaitou 1412's original costume as well as the black feathers (translation of Kaito's family name) being used by Corbeau while he traumatised Kaito with his father's appearance. It just has lots of satisfying symbolism of Kaito finally being able to part of with the weight of his family's tragedy and his father's legacy. Because Kuroba-kun and Kid are just means of getting to know Kaito, Hakuba has no investment in Kid or the Kurobas beyond Kaito himself. Kaito isn't a generic civilian identity that's just another fabrication to hide the criminal secrets or the punniest name for a Japanese phantom thief (because he was literally born and raised for this purpose even with his parents' absences they, including Jii, only enter Kaito's life to help him be Kid for his mother and father's mistakes or lie/escape into another life via Vegas or Poker Face) it's the name of the person Saguru loves and he's his and no matter what name he goes by or what he lies about and hides Saguru will love him, figure it out and find the truth. He'll always ask Kaito for his honest answers and give him Real options to be Himself in it's realest form.
A funny thing happened to me while I was reading your question, I happened to have my playlist on shuffle and "Good Luck, Babe!" started playing, so if you see me rambling a lot, attribute it to that funny coincidence.
You made me think of an arranged/convenient marriage. Not exactly though.
Kaito, who feels hurt and lost after finding out his father was alive and mom knew about it, doesn't want to talk to Jii at all because he can't stand the thought of him taking his father's side (he's not sure if he will or not, so he just chooses not to deal with it), can't ask Aoko or her father for help, doesn't feel worthy of their help.
So he desperately and insecurely chooses Hakuba.
He wants something that feels close.
Hakuba is probably having a cup of tea while enjoying a book, hears knocking on the window in his room, and is surprised to meet Kaito.
"You still want to know my reason?" Kaito doesn't mention KID, he doesn't need to, Hakuba has a bad feeling, something very serious must have happened for someone like KID to even consider revealing his reasons for stealing, he rushes to let him in, offers him tea, but Kaito rejects it.
They both sit in silence.
"I don't know how to start, it's a mess, actually, my life is a mess."
After a couple of minutes, Kaito begins to tell Saguru about his love for magic and his father, the history of KID, and everything else.
The word that Saguru thinks fits well in this situation is “Sad,” and of course, he is so upset.
“Marry me.” The words are out of his mouth before he can process them, it’s not an unfounded request, but it was a proposal made by his emotions.
“I’m sorry if I was so abrupt, still, I mean what I said, I definitely need to think it over better, but I can’t just leave you like this.”
"But how would getting married help?"
"First, because you could adopt a Western name if you wanted, and because it seems very cruel to me that they named you like that… as if your destiny was sealed from the day you were born and you couldn't change it."
"You can think about it, meditate on it if you want, I would never force you to do something you don't want, even if you don't accept me that way, I will watch over you, and I won't let you get hurt again. Take my name, my contacts, and whatever you need to get back on your feet."
Kaito thinks Hakuba is being kind, he doesn't want him to be a second choice or receive a half-hearted love. He has nothing to gain by accepting, instead, Kaito would be the only one to benefit.
