If asked, some people would say that Brittany S. Pierce was as dumb as a box of rocks, but Brittany would just say that it's not true because she has curves and boxes don't so she's as dumb as a sack of rocks. But she'd also say that beauty is in the eye of the beholder and that other people's opinions don't really matter because she's pretty on the inside and then those other people would ask what that had to do with being dumb and she'd change the subject.
So, yeah, Brittany wasn't exactly the most mentally-gifted member of the New Directions, but it didn't matter because they liked her the way she was—a tad touched in the head or not. And, anyway, Santana thought she was a genius, so if Santana thought it then it had to be true, even if nobody else agreed.
Because it was Santana, and Santana knew everything.
But even when her choices, thoughts, and actions were odd or kind of dumb, she always had a way to justify them, even if the justifications only made sense to her. Brittany didn't really need everyone else to understand her, because Santana did, and that's all that mattered.
Even Santana had to admit that Brittany did and said some weird things, like everything that came out of her mouth that involved Lord Tubbington, the one time she swiped deodorant above her upper lip because she liked the smell and wanted to sniff it all day without looking weird, and the 'Lebanese' shirt situation.
(Santana still has that shirt, by the way, and she wears it regularly.)
And although people may laugh at her when she says stupid things and she brushes it off, it really does get to her, even if she doesn't want to show it. Like when her geography class laughed at her because she said the capital of Ohio was 'O,' and it is, because 'O' is the first letter in 'Ohio' and it's a big letter, so it's the capital. Or when Rachel decided that the school musical would be West Side Story and she asked if that was the one with the cats, because wasn't that it? She really remembers cats in that play, but thankfully the rest of the glee club chose to ignore her or just give a brief confused look.
Lord Tubbington, her precious kitty, really does read her diary, even if nobody believes her, and Brittany knows it so that's all that matters. Sometimes Santana doesn't believe her, either, but then Brittany pouts and gives Santana the puppy-dog face and the Latina feels guilty so she agrees. And sometimes Santana lets Brittany touch her boobs as an apology, so that's totally okay because Santana's boobs are amazing.
Brittany really does like Santana, and even though she's not the brightest crayon in the 64-pack with the sharpener, she knows that Santana really does like her, too, and that they'll be friends forever and ever. But Santana's not the only one she likes—she still likes Artie, and Kurt, and Blaine, and Finn, and Puck, and sometimes Rachel when she's not too overbearing and controlling, and Mercedes, and Quinn, and Tina, and Mike, and pretty much everyone else.
She likes Kurt because they really get each other since they're both unicorns (even though Brittany's more of a bicorn) and unicorns are special and should stick together. Santana calls them 'lesbros,' and it's a funny word so Brittany uses it and says that Kurt is her lesbro and they're tight like a pair of panties that are three sizes too small. And because she likes Kurt she also likes Blaine, because Blaine makes him smile and smiling is good so Blaine is good by comparison; at least, that's what she learned in math class when they were going over deductive reasoning.
She still feels really bad about hurting Artie, but they've made up and they're okay with it now and she's happy to call him her friend. She likes wheeling him around and watching him do all those tricks with his wheelchair, and he's a funny guy, so he's a lot of fun to be around. Plus he's a really good singer, and Brittany thinks that if he could walk he'd probably be a really good dancer, too.
Finn is like a genius to Brittany, because even though he gets lost sometimes he still gets more than she does and can really explain it well to her. But he's kind of dumb too and always has that dopey look on his face that just makes Brittany want to pat his head and coo at him like he was a puppy. And when Finn was the quarterback, he was really cute, but when he got kicked off the team he wasn't cute anymore even though he was still a nice guy, and when he was back on the team he was cute again. Even Santana thought so.
Brittany likes Puck because, well… he's good in bed. And he's funny when he's not being a jerk.
Tina and Mike are, like, inseparable, but they're okay and Mike's a really good dancer so he likes to dance with Brittany because she's also a really good dancer and she likes dancing with people who can dance really well (or is it good? Brittany can never remember).
Quinn used to be a part of the Unholy Trinity with Santana and Brittany, but then she got pregnant with Puck's kid and lied and told everyone it was Finn's and she had to quit the Cheerios and then she just went totally downhill and joined the Skanks. But Brittany still likes her, deep down, even though she's kind of crazy and weird and different now. She's still waiting for the day that Quinn rejoins the Cheerios and the Unholy Trinity can be one again.
Brittany thinks Mercedes is kind of loud and intimidating, but she seems like a really good friend and she's patient enough to put up with Brittany's flakiness. And Brittany can't help but laugh just a little whenever Mercedes does her 'hell to the naw' thing and does the head 'thing' that Brittany can't quite do because it takes too much coordination. She likes going shopping with Mercedes and Kurt and sometimes Rachel because it's fun and it's easy to talk about fashion with them, even if she doesn't quite understand what half of the words that come out of Kurt's mouth mean.
Rachel is sort of bossy and mean and controlling and stuff, and sometimes she makes Brittany frown a little because it hurts her feelings, but she knows that it's probably because Rachel never really had any friends and doesn't quite know how to be a good friend without being too much of a mother. At first, Brittany sort of hated her because she got all the solos and was conceited (Santana would be proud of her using a vocab word) and stuck-up, but Rachel mellowed out over time and learned that the world didn't revolve around her—it revolved around the Sun, because that's what the scientists say and they're smart so they're right.
But she likes Santana the best because they've known each other the longest and she really, really, really loves Santana. She's always there to make Brittany feel better and tell her that it's okay and tell her that she's a genius and she's a special unicorn. And Brittany smiles every time because she knows that Santana really cares about her so she hugs her and tells her never to forget that she's a unicorn, too, because she's special and pretty and magical.
Santana is sort of her unicorn, because they're really close and they became unicorns together and they'll always be close forever and ever because unicorns love for life. And even though she's not really that smart and doesn't really get As by working hard (she's pretty sure Sue threatened her teachers with something nasty if Brittany failed any of her classes), she doesn't have to be a genius to know that she loves being a unicorn and she loves being Santana's unicorn even more.