When Matty insists he must go because he promised to bring Kira back for Seer, it shows that Matty still has a strong sense of responsibility to help the people he loves. He is, in other words, fully invested in working toward a common goal and helping others. Matty knows Kira from before—as related in
Gathering Blue—and in his old village, Kira’s disability meant that she would never be considered “lovely.” Matty still retains some of these old prejudices, as his conversation with Leader here shows.