Disclaimer: I don't own Fate/Stay Night or Kamen Rider and they belong to their respected owners.
Archer may have got rid of a few of Berserker's lives but they knew he still had enough lives to kill them all. That's why Saikou knew he had brought out the big guns and upgraded. Saikou Shadow takes out a new lime green and orange Wonder Ride Book.
X-SWORDMAN!
"Eh? You had another one?!" Rin asked.
"Yes, and it's a perfect time to use it," Saikou says.
Saikou Shadow then opens his new Ride Book.
Episode 1: Subete no iro de tatakae!
Shadow places the Ride Book in his Driver's slot. It then presses the button on top of the belt with the bottom of Kougouken's hilt.
SAIKOU HAKKOU!
A large version of the X-Swordman Ride Book appeared behind Shadow.
Get All Colors~! X-Swordman~!
It opened to the transformation and comic panels flew out of the large Wonder Ride Book and started to connect to Saikou Shadow.
"I joined this war as a sword…" Shirou began as comic panels kept connecting to Shadow.
Episode 1: Full Color De Sanjou!
The final piece was his gold faceplate slamming to his helmet finishing Saikou's transformation.
BA-BA-BA-BAN!
Saikou's new form was different, this time it was Shirou's mind was in Saikou Shadow now giving him a physical body. He wore a black and gray bodysuit and his armor had comic style detailed on it. He had purple and black shin guards, with yellow ankle guards, with his black boots having red armor on his toes. He wore yellow armor on his upper thighs and yellow, purple, and red gauntlets with the finger of the black gloves being yellow. He wore purple and yellow armor on his chest with red shoulder guards over armor. He has another layer of purple and yellow armor on the center of his chest in the shape of an octagon with a turquoise 'X' with a white center. He wore a grey helmet with purple and red sides, his faceplate was gold and in the shape of a star with a small 'X' and large cross, and lastly, a blade similar to Seiken Saikou on his forehead. This is Kamen Rider Saikou X-Swordman.
"Now I join it as a swordsman!" Shirou declared as he wields Kougouken Saikou.
Both Rin and Saber are shocked by Shirou's new appearance especially with now having a body and not being a sword. But…
"What's with that flashy look?!" Rin says in a confounded tone.
Berserker just roared and started to swing his weapon at them. Rin got out of the way as Saber and Saikou deal with the demi-god.
"I have to say, Saber, I'm glad to properly fight by your side," Shirou told her.
Saber smiled at the colorful Rider. "I'm glad to fight alongside you as well," Agreeing with him.
"Emiya-kun! Do you have any finisher in that form of yours?" Rin asked him.
"Yes, I do." Saikou close the book and open it back up to the story page
FINISH READING!
He then presses the button on top of the belt with the bottom of Kougouken's hilt.
SAIKOU COLORFUL!
Saikou charges Kougouken Saikou with yellow energy, then creates manga-based projections on the sides of Berserker leaving him only the back, forward, and the empty sky.
Berserker gave Saikou a grunt telling him to bring it.
Saikou flies past Berserker, delivering two white slashes as he goes by, before landing a short distance away. Berserker felt small cuts but felt nothing of it. Not knowing Saikou had delivered another yellow slash back at Berserker connecting with the white slash and made Berserker explode.
"Hmph, got him." Shirou impressed with himself but he knew the battle was not over yet. "Can you keep going, Saber?" He asked her.
"Yes!" Saber nodded as she readied her sword.
"Let me change my form first," Shirou says as he presses the button on top of the belt with the bottom of Kougouken's hilt.
IDOU SAIKOU!
The Wonder Ride Book started to flip pages as the large version of the Ride Book appeared behind him.
UDE SAIKOU! Full Color Goes To Arm!
The armor on Saikou Shadow popped out of him and onto his left arm. His left shoulder had a red, purple, and yellow guard. He had yellow guards on his upper arms and wrists. He a thin turquoise shield attaches to his left arm with purple, red, and yellow coloring in the center of it. On the center of the shield was Saikou X-Swordman's faceplate. This form Saikou X-Swordman Powerful.
Episode 2: Colorful Sword De Zuba-Zuban!
"Oh another form, Shirou?" Saber asked him.
"Yes," His left armed moved. "X-Swordman Powerful."
The girls have noticed that Shirou's mind went to his left arm as that's where the faceplate is.
"Ugh," Rin massages her forehead. "Emiya-kun, I can't handle you or your Rider forms anymore."
Before Shirou could say anything, Berserker roared reminding them of his presence and went to kill Rin with Axe-sword. Saikou ran and slid to stand in front of Rin.
SAIKOU HAKKOU!
Saikou forms a wall of Manga projection from his Powerful X Sword to protect Rin from the Berserker's weapon.
Saikou struggled to hold back the Demi-god's sheer strength but luckily got his attention from slashing him on his back.
"Stay back, Tohsaka, we'll handle it," Shirou told her and joined Saber to finish what Archer started.
AN: Alright, here's part two with Saikou X-Swordman Powerful and F/S N.