Trixie giggled a bit, smirk on her muzzle. "Oh don't worry, Princess Twilight Sparkle. You and your little pet haven't seen the last of Trixie!" she said, giggling and floating a cookbook.
Twilight scrunched. "Trixie, that's how a library works. We give you a book, you keep it for a while, then you give it back. You do understand that, don't you? Giving things back?"
Trixie adopted a mock hurt expression, bringing a hoof up in front of herself. "Oh but of course, Princess. Just like you didn't give back Trixie's Alicorn Amulet, right?"
Twilight's eyes went wide and she stared at the unicorn. "Trixie, that's not at all the same thing and you know-"
Trixie giggled and rolled her eyes, smirk on that muzzle of hers. "Oh it is much too easy to wind you up, Princess. Yes, I will bring your precious book back, you don't need to worry. Please send my regards to Starlight," Trixie said. With a dismissive flick of her hair, Trixie glanced over her shoulder to the princess, smiled, and stepped out of the castle.
Twilight rolled her eyes and closed the door. "Well, we won't need to worry about her for a while. That's a large volume, and I think she-"
"Am I a pet?"
Twilight stopped and looked down to Spike. That dragon was staring at the door, seemingly lost in thought. She scrunched and gave him a gentle nudge. "No, Spike. You're my number one assistant," she said, and flashed the dragon a smile as he looked up to her.
Spike shuddered a little and nodded. "I, uh. I'm gonna... go take a bath, yeah," he said, and scurried into the castle.
Later that night, Spike stepped into the castle library.
"Oh, Spike! How was your bath? Do you feel any better?" Twilight asked, and looked over the top of her book with a smile.
Spike stood there for a few seconds, glancing between him and the door he came in through, before his gaze settled on her. "Twilight, can I... talk to you?"
Twilight floated the book to the side and dipped her head in a nod. She patted the spot next to her and the dragon stepped over. "Is this about what Trixie said earlier?"
Spike nodded and looked up to Twilight. "Am I a-"
Twilight's smile got wider. "You're my Number One Assistant and a proud member of my family, and you shouldn't ever forget that," she says, using a hoof to reach down and rub his spines.
"Oh, yeah, but..." Spike trailed off once more, and his eyes flicked up to the hoof rubbing the top of his head. "I get it... but what does that really mean?"
Twilight tilted her head and her hoof slowed. "It means that I really care about you, you're one of my best friends, and you help me around the castle sometimes."
Spike scrunched this time and looked over to her. "But, you're not my mo-" he paused as he saw the dread starting to overcome Twilight's muzzle. "N-no! I mean, you can't, y'know, replace my mom, but... you kinda do? And you're not my big sister, but... you kinda are, and..." he glanced to the window and let his tail sway.
"What... are we?" Twilight offered, looking down to him with a small smile and a little relieved chuckle, bringing her hoof up to wipe at her eye. "Well to be honest, I don't really know. I mean I know what I think we are but I can't really make that decision on my own," she said, and let out a gentle sigh herself. "Spike, we're honestly really complicated," she said, and looked back down to him.
"That doesn't really-"
"I know it doesn't," Twilight rolled her eyes once more with a smile on her muzzle. She floated Spike up and pulled him into a cuddle. "Spike, you're my little brother, best friend, humble little assistant-" she said, and nuzzled into the top of his head, "-and, well, son, I think, in a way. Or at least I'm some sort of foster mom or, well, surrogate mother, maybe?" she asked herself and trailed off as she looked towards the ceiling.
Spike looked to the pony currently cuddling him. "Uh, Twilight?"
Twi broke from her spell and looked down to him with a smile. "Well, you're all of that, rolled into one. And I'm all of that for you, too. But all of that's kind of hard to..."
"Explain?"
"Well, yes, but also just describe to other creatures, simply. I could say 'guardian' or 'caregiver' but those sound too impersonal. I could say mmm-" her eyes flicked to Spike's for a second, "-mother, but that feels a bit, well, strange. Same for sister, but a bit less, since we haven't ever said it to each other before."
Spike shifts a bit, getting comfortable in her grasp, but still not comfortable with the subject matter. "And 'Number One Assistant'..."
"Sort of... contains all that. Or it did?" Twilight asked. She let out a light chuckle as she considered the dragon in her hooves. "If you'd like to try something different then I'd be happy to."
Spike stayed silent for a few seconds before he looked up to the princess. His guardian, his mo-, his... Twilight. Gathering the courage, he let out a breath. "No, Twilight, I think... Number One Assistant is fine, for now. I just needed to hear that."
Twilight giggled as she pulled Spike closer, and she noticed the young drake wrapped his claws as far around her as he could. "Well I'm glad about that. If you'd like to try different names, we could always do them just inside the castle though."
"I'll keep that in mind, Twilight."
"Thanks, Number One Assistant."
All was still and quiet for a few seconds before Spike glanced back up to her. "By the way, I know you hatched me for your exam, but why did you start taking care of me?"
Twilight smiled and rolled her eyes again. "Well when I was little, Princess Celestia thought I was having trouble making friends-"
"- shocking," he said and glanced up at her. Seeing that irritated look right back at him, Spike paused for just a moment and coughed. "Sorry."
"As I was trying to say..."
Spike smiled as he listened to the story. He was family, a best friend, and a number one assistant. And that was just fine with him.