Chris Rosa

Ms. Rosa's son was a child who lived in pre-war Sanctuary Hills he didn't make it to shelter in time and possibly became a ghoul, Codsworth remarks that he saw him running around in his "Halloween costume" a week early possibly mistaking his appearance for a costume

Background
Before the war, Ms. Rosa's son was presumably one of the neighbors of the Sole Survivor who was restoring an old Corvega with his mother. When the bombs fell, they didn't make it to safety. His mother can be found 210 years later roaming the Commonwealth and Boston with a few other unlucky pre-War citizens turned ghoul, although he is never actually seen in post-war Boston and might have became a ghoul but isn't seen for unknown reasons

After the nukes fell, Codsworth tells the protagonist that he had seen "Ms. Rosa's boy running around in his Halloween costume," implying that Codsworth had seen him as a ghoul and mistaken his ghastly appearance as simply a costume.

Appearances
Ms. Rosa's son appears only in Fallout 4.