Since LupinPurple opened up an RP group for Build, I might as well give off more descriptions about my OC Build rider, Prime.

Name: Prime, given name is Ken Hebika

The name he usually goes with is Prime, but his real name is Ken Hebika. He prefers being called Prime because it's one syllable and it refers to his independence as a hero.

Age: 20

His origins are unknown, nor does he know his origins as well. He only knows his real name and the name given to him by acquaintances. 

Looks: He wears a mute wardrobe; His hairstyle is similar to Kyo Kusanagi's from the King of Fighters series. Actually, his whole looks is basically inspired from Kyo's design; with the black leather jacket, white shirt, chain necklace, chained pants, and fingerless black gloves.

Personality: Pretty much Yusaku Fujiki's from Yu-Gi-Oh VRAINS. Prime believes in the sense of justice and will never stray off its path. He's cold towards others as a pseudo-outcast and often distances himself, believing that no one but him can succeed through. It was until he met a former Faust scientist that his lifestyle began to change...

As he was about to be attacked by a Smash, a mysterious figure quickly saved him with the power of a full bottle. That figure was Lars Tobatu, a scientist under Takumi Katsuragi's aide that no one knew; not even his father or Evolto knew his existence. Lars encouraged Prime to fight against the smash and bring peace to the world. This prompted Prime to wield the Build Driver to fight for justice.

At first, Prime used the Build driver to transform into Kamen Rider Build.

The fullbottles he uses are artificial fullbottles; fullbottles created from Lars' data from other smash other than Misora's power. 

His main form is HebiBlade, by using the Hebi and Ken fullbottles. 

Hebi! Ken! Best Match! Are you ready? The Honing Serpent! HebiBlade!

Besides HebiBlade, Prime has other fullbottles that have their own bestmatches. Here are some of the matches he uses:

TreeGeyser: Tree and Geyser Fullbottles (The Scalding Root Riser)

FrogCello: Frog and Cello Fullbottles (The Amphibious Maestro)

MothNimbus: Nimbus and Moth Fullbottles (The Cloudy Monarch)

PhotonLizard: Lizard and Photon Fullbottles (The Lightforth Reptile)

FalcoNeedle: Falcon and Needle Fullbottles (The Soaring Spiker)

JackWorm: Worm and Jackhammer Fullbottles (The Slimy Earth Rumbler)

Later on, throughout his journey, Prime gets the Raikiri bottle. He can use it in conjunction with the Hebi Fullbottle to form HebiKiri, the Electrifying Serpent.

Prime also gained many upgraded modules thanks to Lars' expertise in the data of Project Build.

Prime Booster: This allowed the user to have enhanced powers of the fullbottles. It enabled fullbottle-based armor parts to happen. This was the basis for a new rider, which I'll get to later.

Single Booster: This allows one fullbottle to be enhanced to its full extent. Its power competes with the FullFullBotle.

Throughout his many battles, Prime was heavily using the Prime booster throughout his fights, so Lars decided to make a new weapon based off the data of the Prime system. His research and analyzation led to the creation of the Prime Driver.

Prime uses the Prime Driver to become Kamen Rider Prime, a name he will permanently keep after obtaining the driver and to distance his affiliation with the Build name. The design of the suit has a Hazard-like appearance, but the details are Brave-like as well.

Hebi! Ken! Prime Match! The Refining Onyx Serpent! HebiBlade Prime!

The Prime Driver has the same functionality of the Prime Booster with the armor parts of the fullbottles, but with a press of a button he can enhance the armor's power for one fullbottle. The default setting is set towards equal power for both bottles.

Also new for the Prime Driver, the Driver uses a Module Core system that allows Prime to use different drivers without getting a new driver. Prime is able to use the full power of the Sclashjellies and Evolbottles, although slightly weaker compared to when used in their respective driver.

Sclash Module: Allows Prime to use Sclashjellies, though it's only exclusive to Sclashjellies. Prime uses the HebiBlade Sclashjelly, which is the first Sclashjelly to have a best match essence in it.

Tsubureru! Nagareru! Afurederu! Prime in HebiBlade!

Evol Module: Allows Prime to use Evolbottles alongside fullbottles. This won't give him Prime Matches, but instead Prime Evolutions. His main form is Evol HebiBlade/Kiri Prime. He utilizes the HebiBlade/Kiri Evolbottle and the Prime Rider System Evolbottle to transform (though it says "Rider System").

HebiBlade/Kiri! Rider System! Evol Prime Match! Hebi! Blade/Kiri! Evol HebiBlade/Kiri Prime! MUAHAHAHAHA~~~

The body changes to one that looks similar to Kamen Rider Evol's, minus the arms and legs. Prime Evolutions also disable him from Parts exchange, but it's compensated with the sheer power of the Evol system.

He also uses the Scorpion (It says "Sasori") Evolbottle to become Evol Sasori Prime.

Sasori! Rider System! Evol Prime Match! Sasori! Sasori! Evol Sasori Prime! MUAHAHAHAHA~~~

Prime can use the adapters originally for the Build Driver and the Hazard Trigger to become Prime Hazard.

Super Prime Match! Uncontrol Switch! Black Hazard! Yabei!

His Super form is Prime Leviathan Hazard Overflow. He uses the Leviathan adapter from Kamen Rider Leviathan.

Leviathan! Overflow! Are You Ready? Rise Up! Feel the Leviathan! Yabei! 

His ultimate form is Prime Destiny. The Destiny Fullbottle is summoned through the Shinko and Kibo fullbottles. 

Shinko! Kibo! Destiny Summon!

Destiny! Are You Ready? The Onyx Primal Justice Warrior! Prime Destiny! Surreal! Truly Surreal!

His finisher is the Destiny Finish; activated by pulling the lever three times (it'll say "first chapter", then "last chapter", and finally "Mugen Chapters").

His movie form:

The movie exclusive items are the Sun and Moon fullbottles and the Eclipse Trigger.

The Sun and Moon bottles can be used by themselves, but will be dangerous if it's with the Eclipse trigger. This is because the trigger isn't fully developed and it needs a user with a high hazard level to master this kind of power.

Of course, Prime masters the Eclipse trigger, which unlocks the full potential of the Sun and Moon bottles.

The Sun bottle has the essence of Black RX.

The Moon Bottle has the essence of Shadow Moon.

The transformation chant for Prime: "Taiyou! Tsuki! Midnight Prime Match! The Aligner of the Sun and Moon! Prime Eclipse!"

Prime Eclipse is a combination of his base form and Hazard armor. Black and White highlights are added on the armor.

After Story:

After Prime leads to a new life and discarding his Prime Driver for the sake of the future, he decides to become Kamen Rider Prime again after witnessing the recent outbreaks of Smash. He steals the Jockey Driver from Kamen Rider Jockey and transforms into Kamen Rider Prime Jockey by using the HebiKiri fullbottle and the Hazard Trigger to override the restrictions placed in the driver.

HebiKiri! Super Disk Drive! Are you ready? Overflow! Electrifying! Dazzling! Welcome Prime Jockey! Yabei!

I think that's pretty much it.

I'm not adding any extra riders here. Just Prime. 

enjoy.