Tobias's story is probably the most tragic of the Animorphs, though they all have their own important and varied reasons to fight. His parents (whom he never thinks/speaks of) have died, he is shunted between relatives who don't give one whit for him, his bullying issues at school - why would he join this war? Humanity has not shown him much kindness, why should he risk his life time after time to free them of this hidden scourge?
At his core, Tobias is a good person
. He buys into the philosophy of freedom with such strength, it's amazing to see the dedication he gives to his adopted cause.
He loses himself in the hawk after finally coming to terms with his future as a nothlit
. What draws him out? A human running for his life. Tobias has the most noble morals of the group, even more so than Cassie (the acknowledged compassionate peacemaker).
All this being said, I wish we could have seen the conversation between Tobias and the human he saves - being a thought-speaking bird and all.