Richard Castle, a renowned mystery novelist, was hunched over his laptop, deeply engrossed in his work when the laughter wafted into his study from the loft's living room.

He glanced over his laptop's screen to find his friends, Kevin Ryan and Javier Esposito, trying to suppress their laughter.

Curiosity piqued, Richard left his work and wandered into the living room to join his chuckling friends.

"What's so funny?" Castle asked, a bemused smile playing on his lips.

Esposito managed to regain his composure first, "We overheard Alexis calling you 'mom' earlier."

"Oh! That," Castle responded, his smile not fading.

"It's not every day you see a fully grown man being called 'mom'," Ryan teased.

"Hey, it's Mother's Day and Alexis doesn't have a mother around," Castle shrugged.

"Her mother, Meredith, doesn't contact her." Castle's expression grew distant as he mentioned his ex-wife.

"I've had to be both mom and dad," Castle admitted, his voice dropping to a murmur.

An understanding silence enveloped the room, as Kevin and Javier let the revelation sink in.

In that silence, Alexis walked in. She was flashing her father one of her brightest smiles, a beautifully wrapped box in her hands.

"Happy Mother's Day, mom." Alexis made her way to Castle, the box outstretched.

Richard accepted the box, his eyes filled with love and pride.

Opening the box, he found a tie with an atrocious pattern of ducks and a heartfelt note from Alexis.

He couldn't help but laugh at the tie, immediately trying it on.

Kevin and Javier laughed along, the earlier tension completely lifted.

Alexis joined in the laughter too, happy that her unusual gift brought joy.

The day continued with more laughter and heartfelt moments as the impromptu Mother's Day celebration went on.

That night, after everyone had gone home, Castle found himself alone in his study.

He looked at the tie again. A chuckle escaped him as he looked at the quirky pattern.

Then, he unfolded the note from Alexis and read it once again.

"I love you, mom. Thank you for being there for me when nobody else was," it read.

Tears welled up in Castle's eyes as the words touched his heart once again.

He realized that though he was teased about being called a 'mom', he wouldn't have it any other way.

For Alexis, he would gladly be both a mom and a dad.

Being able to provide her with the love and care she needed, regardless of traditional roles, mattered to him the most.

He appreciated the bonds they shared, how they navigated through the complexities of family and love together.

After all, being a parent is about more than just being a father or a mother.

It's about being there for your child, loving them unconditionally, and guiding them through life's ups and downs.

Castle neatly folded the note and placed it back into the box, reminding himself of the precious bond he shared with his daughter.

As he returned to his work, a smile graced his face, reflecting the warmth and love he felt for his daughter. A shared warmth they'd continue to navigate through, one day at a time.

After all, in the grand scheme of things, titles don't matter. As long as they had each other, they always had a family.

So, Richard Castle, the renowned mystery novelist, would happily be Alexis' mom on Mother's Day and every other day if it meant seeing that bright smile on his daughter's face.