(Source: The Faerie Queene - Book III, Canto II, by Edmund Spencer)
Being a warrior woman and the daughter of the traditional Arthurian antagonist King Rions, Britomart is a weird character to consider in the context of the wider Arthurian narrative.
Not only because Britomart - being from a Renaissance era Literary Work - means her very existence was never meant to be factored into the French Carmelide narrative, with her father's war against Arthur and Leodegrance.
But also because, intentionally or not, Edmund Spencer ended up creating a very peculiar counterpart to Queen Guinevere:
Guinevere and Britomart are the daughters of Kings Leodegrance and Ryons, respectively. As mentioned, said kings are famously portrayed to be at war with one another that requires Arthur to intercede. But there is another aspect to this: They're both giants in Welsh Myth. King Ryons is based on Rhitta Gawr, a giant associated with Snowdonia, whom Arthur traditionally defeats. While Guinevere's father, in the Welsh Canon, is popularly accepted as Gogfran Gawr (with the name "Leodegrance" being specific to French Canon)
The name "Britomart" itself originates from the Greek Goddess Britomartis. (Which becomes very appropriate with Vulgate Cycle's statement of King Ryons being a descendant of Hercules). Guinevere/Gwenhwyfar, on the other hand, is a cognate of Findabair, daughter of Medb of Irish Mythology.
However, Spencer named his character "Britomart" in favor of a constructed etymology - wherein Britomart is formed from the words "Brita" for Briton and "Mart" for Mars/Martial - which is meant to invoke an association between the martial status of character with the name of Island of Britain. Whilst Guinevere's name, on the other hand, contains the word "Gwen" meaning "White" wherein belies a connection to the old name of Britain before Brutus of Troy conquered it, Albion.
Britomart is interpreted as an allegorical stand-in for Queen Elizabeth I, while Guinevere is theorized by some as surrogate for Queen Eleanor of Aquitaine.
Britomart is one of the few female knights in the entirety of Arthurian Literature. In contrast, Guinevere is portrayed as a sorceress in certain continuities.
Britomart is known as the "Knight of Chastity", while Guinevere is famous for her association with Adultery.
Guinevere is one of the most archaic characters of the Arthurian cast while Britomart is a Post-Malory creation.
If one could make a retelling of the War in Carmelide, taking in Britomart and the Welsh canon into account, then the story becomes a bloody conflict between two warring tribes of giants and their respective chieftains. Hell, you could even insert Yspaddaden and his daughter as a third party to arbitrate between the two factions.
So then, Britomart becomes the powerful warrior princess of the Giants of Snowdonia, leading her father's armies as a frontline champion against Carmelide, becoming a heated enemy and rival to princess Gwenhwyfar ferch Gogfran, who must step up as the defender of her father and her country.
To makes things interesting between Gwenhwyfar and Britomart, Britomart ought to be pure amazon, winning battles through fighting prowess, while Gwenhwyfar leans towards the mystical side of things, having prophetic abilities, other magical powers like shapeshifting and using her cunning to subdue the martial Britomart and the armies of King Rhitta. (Note that, being a giantess, Gwenhwyfar would still have superhuman strength as a given)
And so it goes on for years - seven years according to Leodegrance in Vulgate Merlin - until things get really awkward when King Arthur, Merlin, and the Knights of Logres force their way into the conflict, causing Britomart to balk at having to fight an old friend who, in turn, is seeking Gwenhwyfar's hand in marriage...
