I always thought that である was a way to replace です, or the equivalent of turning "noun" into "being a noun". However, In this ひぐらしのなく頃に fan art piece,魅音 is saying
Which I think means "Let us always be your allies, Satoko"
The でいさせて looks like the causative て form of でいる rather than である. Is that because 魅音 is talking about herself and her friends, sentient beings, and therefore でいる should be uses over である?