Mother Isolde leads a group of non-hostile Children of Atom. She resides with fellow Children of Atom within the Crater of Atom, forming the sole human presence within the Glowing Sea. She is concerned about Brian Virgil's presence nearby, which is seen as an affront to Atom by some of the followers.
There are further dialogue options to learn more about why she and the other followers are in the Glowing Sea, how they survive in the radiation and generally about the Church of the Children of Atom.
Far Harbor (add-on) If the player character has joined the Children of Atom by completing Visions in the Fog, a new dialogue option will appear when first speaking to her. The player character explains that they too are a Child of Atom and she accepts this as truth, though she is slightly suspicious about it.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
Her name is possibly a reference to ISOLDE (an acronym for Isotope Separator On Line DEvice) which is an experimental infrastructure within the CERN complex of nuclear and particle physics experiments.
End of information based on unverified behind the scenes information.