Hiros' pain didn't end after avenging his brother by catching Callaghan. No, it was just the beginning.
Aunt Cass figured this out after the first Christmas following the passing of Tadashi.

All she had done was lit the fire place.

The evening had started so nice too. Hiro was spending the holidays at the quaint little Lucky Cat Café instead of slaving away in his lab or shaking the hands of people at the San-Fransokyo university. Lately, he had been so busy with his latest projects, he would end up sleeping at the school instead of coming home. So when her nephew promised that he would stay the holiday with her, no "if"s, "and"s or "but"s, she was determined to make the best dinner she had made in a long time. Chicken wings.
'Messy, but face-melting bliss' She remembered Hiro had once described it.

So, with everything in place, the night seemed to start off nice, the Christmas tree set up, decorated with ceramic lucky-cats and mini festival lanterns. Hiro and herself were dressed in warm, long sleeved pajamas to fight away the chill that nipped at them ever so slightly. They sat on the floor, near the empty fire place, watching TV as they licked the sauce from they're messy fingers. Aunt Cass actually had a weird thing about eating while on the furniture, but one could say that that was the cost of deliciousness.

As for Baymax, well he had donned a Santa-hat and was currently off in the room next to the den trying to put Mochi in his Christmas sweater Cassandra had made for him.

"Hey, Aunt Cass? It's actually getting pretty cold in here, can he turn up the temperature? Brrrrr!" Hiro dramatically shivered near the end to make his point. Even with the wealth Hiro had brought in with his robots, Aunt Cass still was stingy with the electric and gas bill, so as she got up to turn up the heater, the fireplace caught her eyes.

"What about we have a good ol' Christmas Eve and light up the old fireplace?!" Aunt Cass said enthusiastically. Hiros' eyes lit up with the idea and he nodded his head furiously with joyful agreement to the thought.

Aunt Cass quickly placed some wood and tinder in the fireplace and grabbed her book of matches, the ones she uses to light the burners in the café. Yeah, she's old-fashioned.

"So Hiro, tell me about school today." She began striking the match. 'This one must be faulty.' She thought, as it did not light.

"Well..." He began, she wasn't looking at him, but she could tell he was smiling coyly by the way he was talking."I got over the friction problem I had in my robot... You know Gogo, right? Well, she showed me, the electromagnetic suspension she uses- you know on her bike- well she showed me it can be used on gears, and now that I'm using her method, the robot I'm building can both be strong AND more fuel-efficient , not one or the other." He sighed happily," I might be smart, but I'm definitely not as smart as I am with my friends."

Aunt Cass' cheeks blushed with pride for her nephew, he was so young and so smart, his life was already well-made for him. 'He is so much like his mother and father.' She thought has she tried to strike another match, and it finally worked this time.

"You know Hiro..." His aunt started, as she threw the match in the fireplace and it spread it's flames across the log. "Hiro, you're just like your parents, your mothers smarts, and your dad's determination. Yeah... Your parents were good people, you may not be able to remember them, but they'd definitely remember you... Haha!" She giggled a little." You were talking when you were one!" She laughed once more, this time, a little more sadly," yeah, your mom, my little sister, she was a good person." Cassandra said as she tried not to remember the details of her siblings death.

She was quiet but only for a second," But anyways! Hey, do you want some dessert? I can make whatever you want! Or we could just grab something from the Café downstairs? How does that sound?"

Hiro stared blank-faced at the flames. He had clocked out long before she started talking about dessert.

While aunt Cass was absentmindedly talking her head off, she realized Hiro wasn't responding. She turned around to see Hiro, tears streaming down his face, on his knees with his hands in his lap. She approached him slowly.
She called his name, but still no response. All he would do was mouth his older brothers' name.

She walked closer to him, and sat by him on the floor, feeling the heat on her face. When Hiro still didn't respond, Aunt Cass wrapped her arms around his shoulders and buried her face into the top of his head.

"I'm so so sorry Tadashi..."

Aunt Cass didn't know how to respond, or what to say... She didn't even know if she should respond, was he just talking to the walls? Hiro moved from her grasp, and looked her in the eyes.

"I should've never let you go-o" His voice hitched at the very end.

The cafe owner was mournfully confused, and stayed silent, letting Hiro finish. By this time, Baymax had heard the distressful sounds, and was standing quietly in the room, assessing the situation to find a chance for him to help. Hiro brought his hand to his face to rub his eyes.
"I... I could've stopped you..." He began sobbing loudly. Aunt coddled his head in her chest and patted his back while she teared up with out a sound. His cries grew louder and louder, almost to the point where he was screaming his deceased brothers name. Who wouldn't? He was reliving the scene right there. The fire in the fireplace, his trigger.

Several minutes passed like this, it was probably a half hour that had gone by before Hiro had tired himself out, his head resting in his aunts lap and his body curled up, tears still falling from his eyes. The room was dark and cold and dreary, for Cass had ask Baymax to put the fire out for Hiros' sake. The nurse-bot had sat himself by the woman and her nephew. He wrapped his arms around them and turned on his hearing unit, to warm the quivering boy.

Eventually, Hiro fell asleep. With aunt Cass too tuckered out to even go to her own bed, she fell asleep on the couch, and Baymax carried Hiro to his bed.
'I will contact a therapist in the morning',the marshmallow-like machine thought to himself as he placed his tiny-patient on the bed and stepped into his charging station.

"Good night Hiro." The robot said

"Good night Tadashi."