Fluttershy
Well he was...not very nice. I would say something stronger but Dash has already used a thesaurus of explicits in her rant.

I look over at Dusk whose sat in the corner, staring at the wall, looking rather down. I consider and go into the kitchen and make a hot chocolate for him- it always helps when I'm upset- and go over to him.
"Are you OK?" I ask him, passing him the drink.
"I've been worse," he replies, quietly. That's...a vague answer.
"Are you upset about what Sombra said?" I ask him, watching as his gaze flickers to the ground. "Because, well, it's not true. You're not useless." He gives me a tired look.
"I really wish that was true," he mutters, shoulders slumping. I blink.
"You managed to close the portal," I point out. "So you must be fairly good at magic."
"Sombra still got through though," he counters. "So, really, I can't be that good."

I frown- he really shouldn't take a villain's word for things. He's more than likely trying to get into Dusk's head so he doubts himself. I used to doubt myself a lot but getting friends- and starting to date Dashie- has helped a lot. By the look of things Dusk is in a similar place, he just seems to be used to being alone.
"You still tried," I say. "That's more than most people would do." He doesn't answer that, he just looks thoughtful and takes a sip of the hot chocolate.
"Whatever this is...it's awesome," he comments, quietly. I'm guessing he wants to change the subject...
"You've never had hot chocolate?!" Dash exclaims, bursting her way over, looking shocked. "How??" 
"Um...I've never had chocolate. I don't usually like sweet things..." he replies, picking up a pen and doodling on his hand. 

"Still," I say, going back to the original subject. "None of us think your useless."
"Yeah!" Dash replies, cheerfully. "You're awesome!" I give her a smile: Dusk doesn't look convinced.