For bit of context, while the character in the picture is Britomart, it is not the same Britomart as in FGO. The playable Britomart is actually the daughter of the original Britomart from the Faerie Queene (Britomart Mother as the description calls her).
In OC2, the first major opponent of the chapter is an Altered form of Britomart, who is noted to being the original Britomart. However, Britomart Alter constantly wears her armor in the chapter, meaning that we don't get to see what she looks like under the armor. This is meant to be fan art of what she would potentially look like without the armor.
