Sonja walks until she encounters a echo flower. 

"Behind you..." it said as Sonja looks behind seeing Undyne which she gasped before backing up.

"Seven...Seven human souls...With the power of the seven human souls, our king...King Asgore Dreemurr, will become a god. With that power, Asgore can finally shatter the barrier. He will finally take the surface back from humanity. And give them back the suffering and pain that we have endured. Understand, human?" Sonja has a water bottle behind her which she grips onto it tight and the cap is already off. 

"This is your only chance at redemption. Give up your soul...Or I'll tear it from your body!" Undyne's spear appear as she approaches to Sonja. Sonja dumps the water on herself and transforms herself into a Mermaid. Undyne freezes in her tracks and takes off her helmet and gasps of what she has seen. A soaked and wet Sonja looked up at Umdyne before closing her eyes, knowing that this is the end.

"UNDYNE!!" Sonja's eyes went wide from hearing Monster Kid.

"I'll help you fight!" Monster Kid shouted really excited.

"Yo! You did it! Undyne is right in front of you!" Sonja rolls her eyes and gets into a sitting position.

"Yeah, I noticed." Sonja said sarcastically. 

"Papyrus is right about this...Y-You are a Mermaid." Sonja sighs.

"Why did you attack me? You know that I'm a human right and you're going to kill me with a spear." Sonja asked.

"Well uh..." Undyne rubs the back of her head.

"I guess I didn't noticed the tail when you are swimming when we first encountered with each other." Undyne replied.

"I transform into a Mermaid when I touch water." Sonja clenches her wrist as she dries off her tail with her heating powers. Sonja's tail vanishes, transforming back into legs.

"And you have powers?" Sonja nodded as she stands up.

"I'm so sorry for not recognizing your power right away. I thought Papyrus will crazy but he's telling the truth." Sonja smiled.

"Want to hang out with me? You and I will be BESTIES!" Sonja nodded.

"Great! Meet me at my house! See ya!" Undyne runs off. Sonja smiles and sighs in relief.

"You're hanging out with Undyne?! I'm so jealous right now!" said Monster Kid.

"Don't be but I'll see you again soon." said Sonja as she gives a kiss on Monster Kid's forehead. Monster Kid smiled at Sonja before running off.

"Kids..." said Sonja smiling as she walks off, heading to go see Undyne. 

(Here's Chapter 15. I hope you like it and I don't own anything.)