There exists a disease called
Maeda Jun Syndrome. This kicks in over those that are fortunate and crushes the heart others. Now if a miracle kicks in...
After Toradora End, Ryuuji and Taiga got married and Taiga decides to bear a child.
However, Taiga's health deteriorates, and no cause can be found, while the doctor believes that Taiga should consider abortion, but she turns it down only to perish in childbirth, although having successfully delivered her child. Ryuuji swore to raise his child, Chibi Taiga to maturity.
In Ryuuji's meticulous care, Chibi Taiga grows up to be as cute as her mother and alot less tsun while still as stubburn. This gave Ryuuji great joy in life.
However, Chibi Taiga also has the same disease and her health deteriorites very quickly by the last years of high school, sapping the happiness from Takasu family. This leaves Ryuuji heartbroken and defeated, for it is clear how things will end.
One day, Chibi Taiga insisted on a trip to the ocean which would make such a trip the first time he's taken her out in a while due to his workload. He obliged and they head to a seaside town nearby.
As his child looks on at the ocean from a wheelchair and the people playing over the side of the sea wall, Ryuuji pushed her along the coast and leaving a long trail of footprint and track.
Suddenly, chibi Taiga wanted her father to stand 10 paces away from her and Ryuuji witnessed how his child, haven't walked in a month, stood up with what strength that's left in her as she wobbled about and walked to Ryuuji.
Chibi Taiga called out to her daddy and apologized about how she can't repay him, and that it was enough... and after saying thanks to her father by the time she reached him, her tiny body gave way.
There now remain only the sound of pounding sea and the the wailing sound of Ryuuji...
Spoiler for Afterstory, maybe at episode 22:
Upon hearing the miserable news, Kitamura, Ami and Minori tried to console a devastated Ryuuji, and it took a long time before Ryuuji could get back to reality, but he's a former shell of himself by this point. When Taiga's memorial approaching a few months after chibi Taiga's death, the broken man looked at the photos, thought of what could've been... and fell asleep while lemanting to Taiga about what befell them.
Ryuuji was taken in his dream to his old senior high, but there be no one in sight. He walked about by himself over the school, only to find Kitamura and Taiga over the back of the building where Taiga was turned down by Kitamura. Neither of them can see Ryuuji it seemed.
As Ryuuji watched this drama unfolded itself, Ryuuji questioned if he should've called out Taiga then...
Just as Taiga turned away however, Ryuuji can't really hold his emotions back and he rushed at her.
Taiga turned about and hugged Ryuuji by the waist. She went and said that she was in fear that Ryuuji might've thought that they should've never met.
Ryuuji then reaffirmed his love for Taiga...
And the scene returns to the sound of a crying baby. He couldn't think as he rushed in and saw a pale Taiga. Having called out to her, Taiga didn't open her eyes, but instead took out her pale hand to wipe the tears off of Ryuuji.