"Looks are always deceiving"|
Arcee is a merciless and deadly warrior who's an expert in hand-to-hand combat and one of the best sharpshooters on record. The reason for her ferocity is how much she cares for her fellow Autobots and especially the humans they protect. She knows how fragile her organic companions are, and she's equally aware of the evils the Decepticons are capable of. Peers such as Hot Rod and Springer may be more durable than the humans, but she no less fights to defend them as well. They get in plenty of trouble, so someone has to be the level-headed one.
If concern for her friends is her strength, it's regrettably also her weakness. If anything would happen to, say, Daniel Witwicky, she might just completely lose it.
(Disclaimer: the avatar and profile picture are screenshots from the Transformers G1 show, which belong to Hasbro.)