Britomart is kind of like a medieval Xena, Warrior Princess—but, honestly, she's even better. She's literally a warrior and a princess and combines some pretty amazing fighting skills with dazzling beauty and a romantic heart. Unlike Xena, however, Britomart's armor keeps her gender hidden from the rest of the world, so it's always a dramatic, and often humorous, reveal when Britomart's womanhood becomes known.
When she takes off her helmet in Malbecco's castle, the effect is pretty intense: "Which whenas they beheld, they smiteen were/ With great amazement of so wondrous sight,/ And each on other, and they all on her/ Stood gazing…" (III.ix.23). But just because she doesn't advertise her gender doesn't mean it isn't a crucial part of her identity. As the main hero of Book 3, the Book of Chastity, Britomart's association with proper female sexual conduct is central. But more than that, Britomart is a unique exploration of femininity within The Faerie Queene, where being a woman can often mean you're a evil seductress or a helpless victim (Belphoebe would be the other big exception).
Britomart is definitely neither of things, and her central importance to the poem as whole gives Spenser's handling of women and gender more balance and creativity. Plus, she's just super cool.
Although Britomart is based on the character of Bradagante from Ariosto's Orlando Furioso, she's definitely been given a seriously British makeover in Spenser's retelling. Her name literally means "warlike Briton person," so it's pretty safe to say that being British is a pretty big part of her identity. And sure enough, during the prophesy of Merlin, Britomart learns that she's going to play a pretty big part in British history and begin a line of descendants that will rule Britain for thousands of years to come. No biggie.
Britomart's ties to British rule therefore also connect her to Queen Elizabeth I, the queen during Spenser's lifetime, who, like Britomart, was a woman doing a 'man's' job. Women were not historically as powerful and successful as Elizabeth.
Also like Britomart, Queen Elizabeth was explicitly connected to the virtue of chastity since she never married and made a big show of her status as a virgin. And Britomart is, after all, the main character of Book 3, the Book of Chastity. But Britomart, as we just mentioned, isn't destined to be a virgin. She's destined to marry Arthegall and produce a line of heirs to the British throne. This difference represents a tiny little critique of Queen Elizabeth on Spenser's part for not marrying and thus endangering the succession of the British crown. .
Unlike Belphoebe or Florimell (who are also female characters associated with chastity), Britomart shows us that being chaste and being a powerful, confident warrior can go hand-in-hand. Since chastity involves refraining from sex, we might think of chastity as reserved and quiet, but au contraire, Britomart is both chaste and powerful, totally unafraid to make her presence, and her chastity, an active and visible part of her knighthood.
While Britomart is indisputably fabulous, she's not without her faults. And her biggest fault is her over-reliance on sight and images; looking instead of thinking. We often see her charging knights first and asking questions later, which leads her to attack her very own beloved, Arthegall, before realizing who he is. It also leads her to rather brutally attack Marinell "with so fierce furie and great puissaunce" (III.iv.16) channeling her anguish about loving Arthegall into her sword.
Britomart's impulsive relationship to images also informed her initial love for Arthegall, which is based solely on his appearance in the magic mirror of Merlin. Although their relationship ends up being for the best, Britomart has to learn how to distinguish between being dazzled by an image and contemplating the inner person. This is most explicitly challenged in the house of Busirane at the end of Book III, where Britomart is overly captivated by the murals of rape that cover his walls. When Britomart ought to be acting, she's obsessed with looking. But, the house of Busirane is also the place where she overcomes this fault by eventually finding Amoret and saving her from Busirane's illusions and sorcery.