I don't think it has to do with another girl, but more the fact that she wants to make her lover happy, even if it's at the expense of her happiness. On another note, she also wants to be the ONLY thing that makes him happy
Oooh, thank you! Are all her lyrics so vague and omconspicuous? Haha
It makes more sense if you watch the movie [CASSHERN] beforehand. The same way Be My Last or Beautiful World made sense if you watch the movies they were written for.
The MV for dareka also alludes to the story. I believe the song is sung from the point of view of a father to his son, not between lovers. When she sings "ano ko ga naiteruyo" she refers to a child, not a girl. The kanji ko '子' can be used for 'a girl' or just 'a child' in general. I think that's the poetic part of the lyrics.