A former second in command in the Decepticon ranks and a former scout on Cybertron before the Great Exodus, Arcee is the least physically-imposing of Optimus Prime's crew, but she's also Optimus's most reliable soldier, his go-to gal when he needs someone he can count on. Whatever she lacks in physical form she more than makes up for it in force of personality. She is tough as nails at all times, always ready with a snarky quip even in the face of deactivation, not afraid to bend the rules every now and then, and seemingly never, ever scared.
And while that is largely true, and makes her an incredibly dangerous opponent (especially in melee combat, her specialty), that force of will is also a bit of a front. The loss of a partner back on Cybertron rattled her, leading her towards a path of going it alone. And when she finally started opening up to another partner on Earth, losing him shook her to her core. To then be immediately saddled with the task of being the "guardian" for Jack Darby, one of the locals accidentally caught up in the struggle on Earth, well, let's just say she was less than thrilled about that.
Okay, she was openly verbally hostile to Jack over the whole thing, but her loyalty to Optimus kept her from refusing the task. Like it or not, she's his guardian, and she's going to protect that boy's life... with her own, if she has to.
Slowly, but surely, Arcee is coming around again and facing her past demons. Jack's bravery and ingenuity even when in mortal peril is growing on her. Who knows. He might make a good partner.