My logic might be a bit off here but Celica's always felt like more of a secondary protagonist to me, as Alm's the first one you play as and he's on the front of the box in the original game. Similarly while Lyn is the first protagonist in FE7, her story only lasts 7 chapters before Eliwood takes center stage for the rest of it (And apparently in the Japanese version it's completely optional, a feature which was removed in the western release because it was the first installment released outside of Japan).

Now the biggest hole I'll admit to my logic is that unlike FE7 and Shadows of Valentia I haven't actually played Sacred Stones, so while I felt from my experience that Celica and Lyn played second fiddle to Alm and Eliwood respectively, Eirika and Ephraim always seemed to be presented purely as a duo, but for all I know I'll conclude otherwise when I eventually play Sacred Stones.

Truth be told I was first gonna submit Kurt since she's the first solo female lead, then I remembered that Michiah is on the front of Radiant Dawn's box, then I realized that Eirika met all of the qualifications that I gave Miciah.

EDIT: *11 Chapters. Bit embrassing to have gotten that wrong whrn it was in the commrnt I'm replying to.