Fluffy Demon Shark

The biggest argument against this theory is that no curse spirit was involved so Yuji can't be half curse.

Basically everyone knows at this point that Yuji's parents are Jin Itadori and Kenjaku who possessed the dead body of Kaori Itadori (who from what I have read had some kind of gravity technique I wish Yuji inherited in some form cause gravity techniques are always cool in fights).

However I would like to argue that a curse spirit WAS involved, specifically Sukuna. I have read (I know a surprise in this fandom) in several places that one of Sukuna's fingers was sealed into Yuji long before the series even began. While I can't find the specific chapter that says when exactly this finger was sealed into Yuji, whether it was after or before birth, I did find a quote that might work. In Volume 29 chapter 257, Sukuna talks with Uraume, word for word he says, "Sukuna's fingers were sealed in Yuji Itadori even though he was born as a player in the culling game. I suppose that was necessary to strengthen the vessel." The only way, at least how I see it, Kenjaku could have assured that Yuji would have been able to function as a vessel for Sukuna is if he sealed one of Sukuna's fingers into Yuji was he was still developing in the womb, allowing the unborn Yuji to adapt to it as he developed.

Using this that would, in a way, make Sukuna the missing 3rd parent for this theory, which I also find rather funny.

If Yuji is a half curse then why couldn't he see curse spirits until the start of the series? Because a dormant cursed object was used. The fingers were only awakened at that high school when Sukuna's finger was unsealed and caused a ripple effect to the others. In a way, you could look at it as Yuji's curse energy was laying dormant the same way that Sukuna's fingers were.

Yuji being half-curse also just makes more sense to me as to WHY he is immortal.