I hate that Stannis has the reputation of being without mercy
Admittedly Stannis does himself little favors by his total refusal to do anything to ingratiate himself to others. But when Varys (of all people) says:
Lord Stannis in particular. His claim is the true one, he is known for his prowess as a battle commander, and he is utterly without mercy. There is no creature on earth half so terrifying as a truly just man.
It's a bit wild that the guy who knows everything doesn't know there's more than meets the eye with Stannis. Then there's the famous Catelyn passage:
Stannis pointed his shining sword at his brother. "I am not without mercy," thundered he who was notoriously without mercy.
Cat doesn't know Stannis, so okay fine. So she and Varys don't know that Stannis is the only highborn ruler we see who consistently asks for advice from people around him even if it's an answer he's not gonna like. And they don't know that Stannis does truly believe he has an obligation to both Robert and the realm to claim the crown as Robert's legal.
Also never forget the reaction of the man who never smiles when he saw Davos again post Blackwater.
His eyes were blue pits lost in deep hollows, and the shape of a skull could be seen beneath his face.
Yet when he saw Davos, a faint smile brushed his lips. "So the sea has returned me my knight of the fish and onions."