This happen in EP 29 in original series and EP 30 in my AU.
At the first scene, at Taepung's house...
"Dad! Honestly, I really want to uses that Seiken!" - Taepung said.
"You mean Kaenken Rekka?" - Tori asks.
"Yeah! If I have my grandpa's Seiken, I also want to have my grandma's one!" - Taepung said.
"Well... I think I can let you use Kaenken Rekka but my grandma won't let you do that!" - Tori said.
"What's up?" - Taepung asks.
"Cuz your grandmother said that if you uses Kaenken Rekka, its will make you in trouble!" - Tori said.
"Trouble? But why?" - Taepung asks.
"Kaenken Rekka is the most powerful Seiken of all the Seikens, after Cross Saber! She said that if you uses that, it will put you in danger!" - Tori said.
"Why did you know?" - Taepung asks.
"Because I have a talk with your grandmother at yesterday about that!" - Tori said.
Flashback...
"What? Let your son uses Kaenken Rekka?" - Mari surprise.
"Yeah mom! I know that you won't let him uses it but please, I want to teach him how to use Kaenken Rekka!" - Tori said.
"Uh... Tori! I afraid that I cannot let Taepung uses that Seiken!" - Mari said.
"But why?" - Tori asks.
"Because if he uses that Seiken, his life will be in danger cuz Kaenken Rekka have a enormous power and according to Sophia, Kaenken Rekka is the second strongest Seiken in 11 Seikens after Cross Saber!" - Mari said.
"Huh? But why?" - Tori asks.
"Aiss! Tori! That's not the only reason I don't want Taepung uses Kaenken Rekka! The real reason is..." - Then Mari said the real reason why she doesn't let Taepung uses Kaenken Rekka. After hear his mother say that, he surprised.
Back to current...
"That's the reason she doesn't let you uses that Seiken, Taepung! You have to understand her. She did it because of your safety!" - Tori said
"Okay! I understand!!!" - Taepung deficit.
And everything after like in EP 29 of the original series, also the fight with Anders.
"Well then, what should we do now?" - Perseus asks.
"I will try!" - Taepung uses his Suiseiken Nagare and attack the Slide Anders but not working.
"What? My water power is not working with it!" - Taepung said.
"No, wait! Water is not working with it! It made from plastic and its can be melted under the high temperature!" - Apollo said.
"Apollo! You're the Guardian of Sun, you can use your sun heat wave to make it melt!" - Taepung said.
"Great idea!!" - Then Apollo uses his Solar Excalibur to create the sun ray and shined to the Anders and it start melting.
"Hey! It's work! Ahhh!!" - Apollo still not finish his sentence, The Anders has wiped him out.
"Apollo!!" - Taepung and Perseus said when they see Apollo got wiped out.
"God damn it!!" - Taepung start turn into demon mode and attack the Anders but it has wiped him and Perseus out.
"Hahahahaha!!! You think you can defeat my Anders, you weak?" - The new Anders named Nemesis Anders said.
"Hehehehehe!!!"
"What? Why did you laughing?" - Nemesis start scare when he hear Taepung laughing in evil.
"Weak? We don't think so! Although I can be weak in some points, but basically, I have V-Pups and I am also a swordsman of Kim clans!" - Taepung said.
"Uh... Taepung?" - Apollo and Perseus said.
"Although you're a swordsman but with the Anders like us, you still weak as usual!" - Nemesis said.
"Oh! You think so? Sorry but we didn't like you think! Although I have weakness, but I have friends and family, who always support me! The Anders like you can't be understand!"
"Also, if this Anders afraid of high temperature, I will melt it but not with water power. I will MELT it by the power of the FLAME of anger, bravery and chivalry of a swordsman!!"
At Taepung's house, something start shine red and fly out the box in Tori's surprise.
"Huh? What is it? No way! Could it be... Taepung! He has awoken the power Kaenken Rekka?" - Tori surprise and then Kaenken Rekka fly to Taepung's site. At Taepung's site, he no emitting dark aura again, now around him is fire and fire. Then the Kaenken Rekka come and flying in front of Taepung's eyes.
"What? What the?" - Nemesis surprise.
"Could it be... I have... awoken your power? The Seiken of fire, Kaenken Rekka?" - Taepung surprise when he see Kaenken Rekka.
"Wow!!"
"Awesome!" - Apollo and Perseus surprise when they see Kaenken Rekka.
"Kaenken Rekka! If you have chosen as your new master, I won't let you, my grandma and everyone disappointed! Let's fight together!" - Then Taepung hold the Seiken and its upgraded.
Its look like this:

"Wow! Unbelievable!!" - Apollo said.
"Oh! Interesting!! Slide Anders, attack them!!" - Nemesis command his Anders to attack but Taepung with upgraded Kaenken Rekka has blocked it and wiped out. Then the Seiken start create fire power around it and Taepung. It's won't be the problem if Taepung still okay but its is the problem. Taepung's eyes turns into red at the left and he now looks like a demon want to tearing the Anders body into pieces.
"Hey Taepung! You okay bro?" - Perseus asks.
"I think not! Look!" - Apollo said and point at Taepung who is now become the real demon.
"I see something looks like a cyan big claw next to him! What is it?" - Perseus asks.
"Don't know!" - Apollo said.
"Hey you! Don't make me scare!" - Nemesis said. Later, Taepung rush to Slide Anders and attack it with a fire slash and its start melting a little.
"Holy crap!!" - Nemesis, Apollo and Perseus surprise.
At Taepung's house basement...
"Oh no! Big trouble! He awoke its power and now he has awoken Primitive Dragon's fury nature!" - Tori said.
At Taepung's site, he keep attacking the Anders its start melting more and more.
"And now, finish attack!" - Taepung ready for his finish attack.
"Fiery Dragon Finish!" - Taepung create a big red machine dragon to attack the Anders, then he rush to the dragon and its turn back into Kaenken Rekka and create cross slashes and a rage pass through slash and finish the Anders.
"I will remember this!!" - Nemesis fly to the sky and disappear.
"Taepung!!" - Apollo, Perseus and everyone runs to Taepung now returned to normal.
"You okay??" - Everyone asks
"I'm...fine...!" - Taepung exhausted and then collapse slowly and fainted.
"Taepung!!! Taepung! Taepung! Wake up!!!" - Everyone shaking Taepung but he doesn't wake up.
"I think we should bring him back home to let him rest! Apollo! Bring everyone back home! I will bring Taepung back to home!" - Perseus said.
"Okay! Got it!" - Apollo said and then he brings others and fly away and Perseus brings Taepung back.
Later...
"Uh...."
"Taepung! You woke up?" - The V-Pups when they see Taepung wake up, they really happy.
"Uh... Where am I?" - Taepung asks when he wake up in another place.
"You don't remember this place? This is North Headquarter!" - Mari said.
"North Headquarter? How did I get here?" - Taepung asks.
"Well... I want to bring you back home but she has called me and told me bring you here!" - Perseus said.
"That's true, grandma?" - Taepung asks.
"Yeah! I told Perseus bring you here cuz I want to tell you this!" - Mari said.
"What's up?" - Taepung asks.
"The reason I don't let you uses Kaenken Rekka because of that!" - Mari said.
"What? Why you didn't let me use Kaenken Rekka?" - Taepung asks.
"Well... Because... Since Primitive Dragon become one part of your body, I've thought that you will be okay cuz Primitive Dragon has been tamed by Kento and Sophia but a little of its fury nature still remain of it." - Mari said.
"I know it! But its didn't awake its fury nature when stay and fight with and my V-Pups!" - Taepung said.
"That's because you used Suiseiken Nagare all the time. Primitive Dragon is a Level 2 Card and its element is fire. That's mean its will awake its fury nature when you use fire power! Or rather...if you awake Kaenken Rekka, that's mean you have used fire power and touch to its fury nature!" - Mari said.
"Really?" - Taepung asks.
"Yeah! Once you have awake Primitive Dragon's fury, its will turn the person that its chosen into a demon and cannot control themself!" - Mari said.
"It's like me when I use your Seiken! I have turn into demon and can't control myself!" - Taepung said.
"Yeah! I have told your father about that but I don't know why he let you use that!" - Mari said.
"No! It's not his fault! It's my fault! I have told him to teach me how to use Kaenken Rekka and I cannot control my anger so I have awoke its and didn't think about the consequences it will cause for me when using it!" - Taepung said.
"Don't worry! It's okay! I also like you when I was young! When I used Primitive Dragon with Kaenken Rekka, I cannot control myself until I get Elemental Dragon!" - Mari said.
"Okay! I know! Can I go home now?" - Taepung said.
"Yeah! But keep rest! You just awakened so you need to rest more!" - Mari said.
"Okay! Thanks and bye, grandma!" - Taepung said.
"Wait!" - Mari said.
"What happens, grandma?" - Taepung asks.
"Well... I know a person can turn Kaenken Rekka into a device! Follow me!" - Then Mari give Taepung and his V-Pups to a door and after go into, its lead to... West Headquarter!
"Wow! I didn't think about that you have a door lead to West Headquarter!" - Taepung said.
"Oh Mari, Taepung! You come?" - Kento asks
"Hi Kento! I need your help!" - Mari said.
"What's up?" - Kento asks.
"Well Kento! Can you help me turn Kaenken Rekka into device mode like Pada and Pilseung hyung's one?" - Taepung asks.
"Oh yes! Give it to me!" - Kento said and Taepung give Kaenken Rekka for her. Later, Kaenken Rekka has turn into a the Seiken sword guard and fly around Taepung.
"Wow! Thanks, Kento!" - Taepung said.
"No problem! The way to summon it is really easy! Just hold it and bring it sideways or other ways like up, down to summon it! Try it!" - Kento said.
"Okay!" - Then Taepung hold the sword guard and bring it sideways, its has turn back into Kaenken Rekka.
"Wow! Awesome!! But how I can turn it back to its device mode?" - Taepung said.
"Drop it!" - Kento said.
"What? Drop it?" - Taepung asks.
"Just do!" - Kento said.
"Okay!" - Taepung drop it out and its turn back into device mode.
"Or more easier, get it a name and say its name and then say "Device mode!" - Kento said.
"Okay! I will do it when I come home! Now I have to go home! My parents are waiting me!" - Taepung said.
"Okay! See you later!" - Mari and Kento said.
"Bye!" - Taepung said and with his V-Pups go back home.
MysticalStoryWeaver That's the story how Taepung got Kaenken Rekka. Hope you like it!