Una's name, which means "oneness," is deceptively simple—kind of like her. While "one" might seem like a pretty basic, uninspired name, it actually encapsulates two qualities that Una embodies: the unity of Truth and the One true church… and yes, they are related. Una's gender also anticipates a common binary that will continue to come up in the text: women are either the embodiment of total purity, like Una ("So pure and innocent, as that same lambe,/ She was in life and every virtuous lore" (I.i.5)) or they embody a perception that women have a dangerous, and potentially uncontrollable, sexuality, like Una's evil counterpart, Duessa.
Sadly, this extremely reductive binary often informed how women were viewed during Spenser's time… and, unfortunately, at times even today.
In contrast to Redcrosse's mistake-making, wavering, and general cluelessness, Una is rock solid from the get-go: sure of herself and her morals, sure of her love for Redcrosse and her family, and sure of her dedication to Christianity. So, Una embodies the importance of having unity and harmony within yourself, of not letting all your different fears and desires pull you in lots of different directions (sometimes literally, as in the case of poor Redcrosse who spends so much time wandering around).
Related to Una's sense of self is her consistency, she's "one" because you can always count on her to be the same Una you know and love—no flakiness here. She's described as "The fairest Una, [the king's] onely daughter deare,/ His onely daughter, and his only heir" (I.xii.21) and you can see from how Spenser spells "onely" that he really, really wants to emphasize her unity and uniqueness.
Again, this contrasts pretty strongly with Redcrosse, who seems to constantly change his mind about himself and about the people's he's with. It's also in stark contrast to both the false-Una Archimago conjures to trick Redcrosse and the witch, Duessa, whose name means "doubling" and who is constantly pretending to be people she isn't (for more see "Duessa"). The association between truth and unity go way back to Classical philosophy, and you can probably see why. Truth should be a certain, single thing, not lots of different things that might contradict one another.
Una represents the One True Church, i.e. the Protestant Church. Her association with truth and solidity obviously come in handy here, but there are other attributes that align Una squarely with Christianity. She is first seen riding a donkey, which is the animal Jesus used to enter Jerusalem. The donkey, unlike the horse, is a pretty simple animal and thus shows that Una, like Jesus, is humble.
Humility was a particularly important Christian quality for Protestants, who valued a humble and simple lifestyle. Una, therefore, is also dressed quite simply, wearing black for most of the poem, and then switching to white at the end. No colors, no frills, no ruffles... get it? The lamb that is her companion is a symbol for Jesus, both because lambs are innocent and sweet, but also because they used to be sacrificed to God before Jesus came and made himself a sacrificial lamb.
The lamb's innocence also reminds us of Una's virginity, which was considered to be an important quality for a woman to have in Spenser's time and which further separates her from the lustful Duessa. Finally, Una is also associated with none other than Queen Elizabeth I, who was the head of the Church of England (Spenser's one, true Church), was famed for her virginity, and was known to dress in black and white. Still think Una's super simple? We didn't think so.