“[BRITTANY]: Female 18 to play younger (15-17) Caucasian. Very attractive, charismatic and confident. Possibly a bit edgy with a fiery personality, Brittany is a born entertainer. Strong singing skills are a plus, but not mandatory. SUPPORTING”
The site speculates that this might be the character of Alison Blaire a.k.a. Dazzler who was hinted at in a deleted scene from X-Men: Apocalypse. They pointed out that the version that we might be getting is the Ultimate Marvel version of Dazzler.