Fire
Undyne was fire, and Alphys was caught up in the inferno.
Undyne's hair was a beautiful blazing red and always took Alphys's breath away, especially when she stepped into the sunshine, which set her hair alight, adding a beautiful golden glow to the already brilliant crimson.
Her eyes burned with determination, captivating Alphys with her intensity and leaving her unable to look away.
She was wild and carefree, sometimes accidentally leaving destruction in her wake(especially when she tried to cook). But she never gave up, determined to do her best at everything. She was full of passion, full of life, full of light, and that's what Alphys loved the best about her.
One look from Undyne could ignite a flame in Alphys, making her desire even more of the wild beauty that was Undyne, and every kiss, every touch, only made the fire burn stronger, burning Alphys straight to her very soul.
And Undyne's fire wasn't just passion, it was a soft gentle glow of comfort as well.
Whenever Alphys was sad or scared or hurting, she could go to Undyne, and Undyne would take her in her arms, comforting her and filling her with warmth from the inside out.
Undyne loved spoiling her girlfriend and would create sweet surprises to make her happy, like showing up at her work with coffee and her favorite doughnut, or (attempting) to make her a romantic dinner, or hiding little presents in surprising places just to make Alphys smile.
Even in the dead of winter, Undyne made Alphys feel warm.
And she never wanted that fire to go out.
...
"Hey, Alphy. You've been rather quiet." Undyne said suddenly, bringing Alphys out of her thoughts. The two were sitting on the couch, sipping hot chocolate in front of the fireplace as a snowstorm blew outside the window.
"Oh! Sorry, Undyne! I was just thinking." Alphys told her.
"About?" Undyne asked curiously.
Alphys blushed. "You."
Undyne's eye sparked playfully. "Oh?"
Alphys nodded and leaned up, pressing a kiss to Undyne's lips and igniting a fire of her own.