Rachel and Tobias, definitely. Not just together, but as individuals too.
I envy Rachel for being able to freely lose herself in the adrenaline, and seemingly not care about what happens in the process. She's fierce, beautiful, and strong. People's admiration of her went seemingly unnoticed - not because she was shallow, vain, or flaky. But because she has her own share of insecurities too. No matter what kind of a game face she puts on and how many times she says, "Let's do it!", she is always aware of that fear, and yet, she doesn't let it bother her until after the mission's over.
The Star Wars nerd in me compares her to Anakin - dangerously close to the dark side. And yet, she would never in a million years give into it.
It really angered me to learn that she died, although I think part of that was because I accidentally stumbled upon the spoiler before I'd reached that point in the books.
Tobias is, well... I've fallen in love with people like him in real life. He's sensitive, and remained surprisingly not jaded throughout the series, despite, as most of the characters put it at some point or another, being the one who has sacrificed and lost the most in the war with the Yeerks. He lost his family, then his human body, and finally, Rachel. He had to feel terribly betrayed by the person who saved him from getting toilet dunked when they were still normal teens.
I love how of all the characters, he has the most secret plot twists as well. He's given the power of morphing back by the Ellimist, but his hawk form remains his default. He meets his mother, finds out who his father (and uncle) are. I've always found it amazing how, technically, he's half-Andalite. Even if Elfangor allowed himself to be trapped in human morph to stay with Loren.
It was randomly remembering that that made me want to read the books again, as I never actually finished them back in the day.
