Is a girl who reached puberty and did not fast, then repented, required to make up the missed days along with feeding the poor for them, and is the feeding waived if she is unable to afford it?

You must offer sincere repentance and perform many good deeds. You must also make up the days of Ramadan that you missed after reaching puberty. If you delay making up these days without a valid excuse until the next Ramadan arrives, then you owe expiation (kaffarah) of feeding one poor person a "mudd" of food for each day. This expiation is waived if you are unable to feed, and it is also waived due to an excuse, such as ignorance of the impermissibility of delaying the make-up days.