An extremely adorable shipping that includes Red(boy) and Yellow(girl).
These two are probably one of the sweetest couples you can find in fanfiction.
Hints in the manga:
Red saves Yellow from a Dratini in R/G/B and promises to her to be the gym leader
In the Yellow Arc, Yellow goes on a quest to find the missing(kidnapped) Red. She's very determined to find him.
Yellow becomes very embarassed when Red(not realizing Yellow was a girl) jokes that they should live together.
In the FRLG Arc, Yellow is thinking about Red, even while unconious.
Red carries Yellow (bridal style) while she is sleeping. (They are then frozen in stone in the same position for several months.
And there are smaller, ssubtle hints throughout the manga.
Their pikachus, Pika(Red's) and Chuchu(Yelow's) mated in the manga and had an egg. Pokemon are supposed to reflect the feelings of their trainers.
Also, this shipping was announced canon by the creators.