Uguu.
Nayuki-san's favourite food...
Ok, I'll give you some hints.
Yuuichi-kun, what's Nayuki-san's favourite food?
It's red and it has leaves.
Wrong.
Hint number 2.
It has grains similar to seeds.
Sesame dumpling!
Hint number 1.
These
Carrot!
Uguu plus quiz
It's persimmon!
I got it!
Where did the hint number 1 go?
Hint number 2.
Grains similar to seeds.
Hint number 1.
It's red and it has leaves.
Yeah.
Yeah.
Hint number 3!
It's both sweet and sour.
Aren't you putting aside just one hint?
Uguisu
Japanese bush warbler (probably a reference to Motosu city's symbols http://en.wikipedia.org/wiki/Motosu,_Gifu)
The answer is strawberry.
It's basically sweet, though.