How about this? It's been a few years since Aizen lost the war, Ichigo's body is 18 years old, and Nel is still controlling it. When Nel first took control of Ichigo's body, the mindset was still that of a young child's, but after a few weeks, it completely adjusted, so Nel's personality is like that of the adult Nel's now. Nel has more or less accepted his/her current body and has adjusted pretty well to Ichigo's life and even bought an apartment to live in with his 'daughter.'
Ichigo, on the other hand, is still adjusting. A few months ago, Kisuke created a gigai that works with hollows and arrancars, and made it similar to the first one he gave to Rukia. Similar, as in, it won't get rid of the inhabitant's spiritual powers, but it will turn them into an ordinary human if not regularly taken off, which Ichigo does to avoid creating a chain of fate, but wears it enough to age his/her current body. Also, (since gigai and human bodies seem to work best with those that are physically closest to them) Ichigo's gigai looks like Nel without the mask.
Just before Ichigo got the gigai, Nel moved out of the Kurosaki clinic into a nearby apartment, and teaches martial arts, sword fighting, and some other stuff to make money. After Ichigo got the gigai, Nel adopted the former human, and on the official papers, it says that Ichigo Kurosaki owns the apartment and lives there with his 5 year old daughter, Nel.( Mandatory education does not start until the age 6 in Japan, so Ichigo still has a few months (April-the month Nel was born and the month school starts in Japan) before he/she has to go to school.) Ichigo was told to call Nel 'daddy,' 'dad,' or 'papa,' and after months of prodding, Ichigo finally gave in. Nel was told to call Ichigo 'Nel,' or some nickname in order to avoid anyone being suspicious of them. Nel understood why, and did not hesitate to do so. But, when they're alone, they sometimes call each other by their real names.
Ichigo is also going to the school for Soul Reapers after regular school in order to better understand how to use spiritual pressure, the captains ordered the child to attend.
So, Ichigo is annoyed, especially since Nel's body wasn't potty-trained, so he/she has to re-learn that too.
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