An abbreviation of a line from the intro to an anime called “your lie in April” typical said as a sign of affection and love.

It stands for

Kimi da yo kimi nanda yo. Oshiete kureta Kurayami mo hikaru nara Hoshizora ni naru

And translates to
It was you, it was you! The one who made me realize; if we can make even the darkness shine, it will become a starry sky.

the thing that you don’t have when you’re phones percentage is over 20