Up until his capture, Soundwave's most defining characteristic, aside from his apparent lack of what most would consider an actual face, was his total, and unsettling silence; he could speak but chose not to a testament to his evident skill with keeping secrets. His equally defining characteristic was his unquestionable loyalty and devotion to Megatron, who in turn trusted Soundwave fully and completely, possibly more so than any of his other minions and showing him considerable favor. His loyalty drove Soundwave to maintain a constant vigilance on the Nemesis for any potential conspiracies against his master and alert him of the would-be traitors. Megatron even asked Soundwave to remind him of Airachnid's trustworthiness (which Soundwave willingly obliged by replaying a recording of Airachnid declaring her belief in the need to consider a future without Megatron). Soundwave himself apparently did not trust any of his fellow Decepticons, particularly Starscream and Airachnid, keeping a constant watch on the former (who continuously plotted to take over leadership of the Decepticons) and later enlightening Megatron that Airachnid was not to be trusted. Soundwave diligently followed his orders without question or hesitation, and often succeeded in all his efforts, rewarding him with praise and further trust and favor from Megatron. As long as Soundwave is around, Megatron has nothing to fear, and Megatron is well aware of that fact.
It's not just loyalty to Megatron that makes Soundwave such an effective soldier, however. He is a skilled fighter, being that he was second only to Megatron in the gladiatorial arena of Kaon. He is concise and pragmatic in all of his endeavors. This makes him one of the best hunters on Cybertron. If he doesn't personally harm anyone during an outing, it cannot be considered an act of mercy or decency. It is simply not a part of his mission and not worth considering. Primus help those unfortunate souls who are in the way or are his mission, as he will almost certainly find them and it will not be a pleasant experience when he catches his target, as Ratchet found out.