AN: How is it going, everyone? I am here today with another one-shot and it will be a KR crossover, but one with Code Geass. What would it be? Here is a hint.

"Now, we shall begin the experiment."

Kamen Rider Build: Rabbit-Tank Revolution

Kallen Kozuki immediately rode the motorcycle straight to her home in a hurry. She cannot help but think about what she went through after being kidnapped by scientists and being experimented until she break out. There she got caught up in a fight with monster and Britannia boy in strange armor.

I have to make sure that she is alright… Kallen thought as she soon made her way straight to her home. To her shock, the place is already set on blaze.

Lying on the ground dead are few servants especially her stepmother who Kallen wasn't sure to missed. But the redhead's attention focused on the one who spew flames.

The one who set everything on fire was a metallic like creature with orange sphere with spots for head and arm resembled a flamethrower. This is the Burn Smash.

"This cannot be real…" Kallen whispered in shock and horror. "K-Kaa-san?"

The Burn Smash turned around and looking at Kallen who is shocked before walking toward her.

"Kaa-san, it's me. Kallen…" The redhead spoke nervous.

But it wasn't for the intervened of a certain raven hair teen who got her out of the way. This got her out of her shock especially after she was certain that she ditch the guy by knocked him unconscious on the way to her house.

"W-What are you doing here?" Kallen asked to Lelouch who studied the Burn Smash carefully.

"So she did become Smash like I thought…" Lelouch said before he takes out a buckle with two slots for bottles and a grip handle bar on the left. He placed the Built Driver on his waist as belt formed around his waist.

"Wait! Please don't kill her!" Pleaded Kallen to Lelouch. She never thought that she would be pleading to a Britannia boy to spare her own mother despite her issue with her.

"Don't worry. As long as I remove the Smash essence, she should be back to normal." Lelouch assured to her. He then takes out the two bottles from his pockets.

He then insert them straight into his Build Driver.

RABBIT!

TANK!

BEST MATCH!

ARE YOU READY?

A quick hand moved the black lever at the side, spinning it quickly before Lelouch gaze hardened towards the Burn Smash.

"Henshin!" shouted Lelouch.

"HAGANE NO MOONSAULT! RABBITTANK! YEAHHH!"

The large wheel upon the right hand side of the belt buckle whirled with every spin of the red handled lever at the side, two smaller silver gears connecting and rotating too. Out of those two gears, a pair of pipes flew out as the rhythmic sounds of metal colliding rung out, forming a strange rectangle around Lelouch upon the ground. The pipes moved skyward, forming strange pieces in the air, with red in the front and blue behind, making various pieces as the Britannia teen was covered in a black bodysuit, the two halves in front and behind coming together upon it to form diagonal stripes upon his body.

His left hand was covered in blue armor to contrast the glowing crimson that covered his left. The opposite was true for his legs, with the addition of a white spiral around red leg, the left. His chest piece was the same, with red being on the top and blue underneath, but the helmet, covering a similar pattern was a small bit more interesting. Two compound eyes looked forward, the right in blue, and the left in red, but it was the shape that intrigued. While they both had antenna, the right had it in the shape of a tank barrel while the other had the ear of a rabbit, a simple eye and nose added to the left eye.

This is Kamen Rider Build RabbitTank form.

Build soon charged forward as he delivered a flying kick and it sent the Burn Smash back. Build soon followed up with punches to further pushed the Smash backward.

Before Build could continue, he received a message coming from Nunnally through the communication device in his helmet. He activated the voice message from his younger sister to see what she need.

"Nunnally! What is the matter?" Build asked confused.

"Big brother! This is bad!" Nunnally said, making Build feeling worried. "I was monitoring the Smash. The computer tell me that… Kallen's mother is only level 1." She informed to her older brother.

This caused Build to stop and looking at Burn Smash. He remember reading something from Takumi Katsuragi's journal about Smash level 1.

Hazard Level 1 only occur when the person in sublimated health and…

"What!? Then that means… she could die if I removed the essence." Build said in shocked. Kallen also heard the news and her eyes wide in shock.

Kaa-san will die…? Kallen thought. Despite all of her rough treatment toward her mother who she despise for staying with her so-called father, her mother… is always there when she need her. Always there…? She is always there? She suddenly start have a shocking realization. 'She is always by my side… That is why she…'

The Burn Smash blows flames at Build and sending him flying backward, making him hit straight to the wall. Kallen soon runs toward her mom and hold onto her arm.

"Kaa-san, please stop this!" Kallen pleaded desperation for the first time. "Don't you recognized me!? It's me, Kallen! I understand why you stay with me all this time! Please come back to me! I am so sorry! I'm sorry!" she cried.

The Burn Smash growl before knocking Kallen away and she rolling across the floor.

"Kallen!" Shouted Build before he saw the Burn Smash about to kill her own daughter. Build is going to run to save Kallen… but he find out that he doesn't need to.

The flame only missed Kallen when the Burn Smash somehow altered the direction of her flame to herself and making her collapsed to the floor. Burn Smash trying to fired once again but keep hitting herself to further her pain.

"Kaa-san…" Kallen whispered.

"Turning into a Smash should cause losing of identity…" Build said, "But she hurt herself, avoiding to hurting you."

This made Kallen looks confused at him before looking back at her mother who continued to not firing at her and hitting herself.

"Why…?" Kallen croaked in confused. "Why would she do that after everything I put her through? Shouldn't she despise me for everything I did?"

Build looks at Kallen before he respond, "Because she is your mother and it's her duty to protect her daughter. Wanting the best for her children."

Kallen soon starting to think back to the time before Britannia conquer Japan and how she used to spend time with her mother and older brother Naoto. She then think about the time after Britannia conquer Japan, her mother spend her time as a servant and being treated roughly by her and her step-mother.

"Is there no way to save her?" Kallen asked.

Build remained silent as this give Kallen the bitter truth that her mother couldn't be saved. She soon then looks up at Build and swallow her pride to ask him doing something important.

"At least… turned her back to normal, please… Turned my kaa-san back to normal!" Kallen requested quietly. "Please!"

Build looks at her then at the Burn Smash. He then take out a green bottle before he replaced the tank bottle with it. He once again crank up the Build Driver.

RABBIT! VACUUM! ARE YOU READY?

As the rhythmic sounds of metal colliding rung out, forming a strange rectangle around Build upon the ground. The pipes moved skyward, forming strange pieces in the air, with red in the front and light green behind, making various pieces, the two halves in front and behind coming together upon it to form.

He still donned his Rabbit Halfbody, but his Tank Halfbody is replace with light green. The right eye now resembled a vacuum cleaner. His right hand now resembled that of vacuum.

Build soon absorbed the flame around the area and they vanished complete except for the burned up corpses. The Burn Smash got up and firing fireballs at Build, but they were sucked up by Build's vacuum like hand.

It wasn't long until Build finished drawing the flame as the Burn Smash collapsed to the ground after being weakened.

Build then pressed a button on his right shoulder and it released all of the flame he sucked from the Smash into the air. He then got out two more bottles- a brown and clear color.

GORILLA!

DIAMOND!

BEST MATCH!

ARE YOU READY?

The same thing happens again to Build before he donned new armor that is brown and clear color. His left visor resembled the face of gorilla. His right visor resembled that of diamond. His left arm shaped like that of a gorilla's arm with large fist. His right arm has a diamond for shoulder pad.

KAGAYAKI NO DESTROYER! GORILLAMOND! YEAHHH!

Build raised his right hand into the fire twister he created as result of multiple diamond materialized. He then cranked the handlebar again.

GORILLAMOND!

YEAHHH!

The diamonds soon swirling around Build as he continued cranking the lever.

READY GO!

Build soon swing his gorilla arm forward and send the diamonds at the Burn Smash.

VORTEX FINISHER! YEAHHH!

Kallen watching Build who charged forward at her mother who is being spinning in a diamond tornado now.

Suddenly her eyes wide in shock to see her mother- Sena Kozuki dropped to the ground. Build moved above her and protecting her from the tornado with his BLD Prism Glove.

"Kaa-san!" Shouted Kallen before she ran to her mother's side. She take her up to her arms close and watched her stirring up.

"K-Kallen…" Sena whispered. Kallen is both worried and shock at how her mother was alive in the first place when she thought she is gone.

"B-But how is she still-"

"This is all I can do for her…" Build said, still holding up the diamond tornado.

"Kallen… I'm so sorry." Sena apologized.

"What the hell you're talking about?!" Kallen choked with tears. "I am the one who should be sorry for all those time when you work to stay by my side!? You didn't have to do it!" she continued with tears rolling down her eyes.

"Even so, I am your mother… I wanted the best for you." Said Sena. "As a mother's duty, it to ensure that her children have the best quality of life possible, regardless of what happens to me. As a Britannian, you have the world at your fingertip. There is almost nothing beyond your reach… you would have been safe…"

"but… I can't be happy as a Britannian, Kaa-san. I just can't. I am Japanese and I am proud of it." Kallen croaked. "Not after I lost Naoto-niichan and not being able to fulfill his dream of freeing Japan."

This made Sena looks sad at her especially mention her late son. This made her realized how much she neglect her daughter with what she wanted.

"Oh Kallen…" Sena said sadly before she continued glowing. "I didn't think of what you wanted…"

"Kaa-san… it's alright… I am still sorry for all this time and I cannot make up for what happen…" Kallen choke said.

"No matter what happened… I always love you. Try your best, Kallen." Sena said closed her eyes and faded away into dusts.

"Kaa-san…" Kallen said with a hiccup as she watching her own mother faded before her presence.

Build soon slammed the diamond storm away from them and the body of Burn Smash lying on the ground. He takes out an empty bottle and used to take in the essence of the Flame Smash before it vanished completely.

Build soon undo his transformation and he looks at the bottle in his hand. He soon hearing sobbing sound and looks at Kallen who is still where she was.

"Come on, we should get out of here." Lelouch said.

"What is the point?" Kallen asked lament. "I lost both of them…"

Suddenly she felt she was grabbed in the air and seeing Lelouch glaring at her. "Are you really okay with it?! There is no doubt that Britannian official will come soon and there are those people who experimented on you! It mean that you're still in danger!" Lelouch stated angrily. "And that's fine? Is that what your Kaa-san wanted for you?! What about your brother's dream of freeing Japan? Don't they mean anything to you?"

Kallen still don't know what to say after she was released by Lelouch. She looks back at her burned down mansion. After losing both her brother and mother, she has nothing left here. She doesn't even wants to live with the man who abandoned her mother.

Kallen soon got up from the ground and soon find her way to Lelouch who sits on the motorcycle. He offered her a helmet.

Kallen take the helmet and strapped it onto her head before sitting behind Lelouch. Lelouch starts the bike as the two left the scene before anyone come to the burned down Stadtfeld mansion.

To be continued….

AN: This is for my experiment attempt with writing a Kamen Rider Build crossover one-shot. This idea didn't come to me originally, but jetslinger333 who thought about it and want me to try it out first. Yeah, Lelouch and Kallen are going to be Sento and Ryuuga duo here. Not to mention I can totally see her being a badass Cross-Z! Nunnally will have more vital role here than her canon self. Anyway, be sure to review or message me for any questions, comments, or concern. And don't be afraid to look at my other stories.