(ignore the spoilers if you don't want to spoil anything from the anime)
Canon characters are allowed to be used because this rp takes place in a alternate universe meaning characters can be the same but act different/doing different things example Saber not being Shiro's servant also on the note of relationships. You can have relationships but keep it pg13 the setting is the same as fate/stay nights for those of you who don't know it here's the setting there has been this war for the holy grail which grants the winner a wish this is the 5th war for the holy grail and each time there are a number of magi (masters) who summon their servants in this instance one of each class (classes are as follows Saber, Archer, Rider, Assassin, Berserker, Lancer, and Caster. Will go more into detail on the classes later) the exception to this would be Gilgamesh in this instance he has no master and is pretty much his own class since he can summon any of his weapons at any given time. 
Back to the setting each master and servant fight against the other masters and servants sometimes forging short term alliances with other masters and servants to defeat a more difficult master/servant also the world that they fight in is created by the holy grail which is separate from the real world. A master/servant will be disqualified if their master/servant is killed in a fight since a master cannot properly defend themselves without a servant and a servant cannot exist without their master. Before I forget a servant is a hero from the past and the holy grail pretty much creates a copy of them to fight this copy will usually be them at their most heroic state which can gender bend them most obvious example would be [hider=Spoiler] Saber being King Arthur although Saber is a she in the anime [/hider] though they do not have to be a real hero from history for example [hider=Spoiler] Archer is the main character from the anime from a alternate timeline [/hider] .
Masters also have 3 command spells which can force their servants to follow a order their servant though how strong it is on the servant depends on how specific it is pretty much a order like obey everything I say is a pretty useless order but a order like help Berserker kill Saber would be pretty powerful due to how specific it is and if the servant tries to resist such a order they would feel intense pain if a master uses all 3 they will lose their control over their servant but the servant can still help them if the bound between the two is strong enough. Another note is that masters call their servants by their class usually since others knowing their true name can lead to them being easily defeated now onto the classes 

Saber: Servants placed within this class are agile and powerful melee warriors. It is commonly assumed to be the best class overall, with high ratings in all categories. It is one of the three Knight classes, together with Lancer and Archer and also possesses the Riding skill like the Rider class.

Lancer: Servants placed within this class are very agile and skilled with long-range melee weapons, such as spears, lances, etc. It is one of the three Knight classes, together with Archer and Saber.

Archer: Servants placed within this class are usually proficient with projectiles and can survive longer without a Master nearby, thanks to their special ability Independent Action; the strongest Archers can be difficult to control at times, in fact, due to their near-total independence from their "Masters". It is one of the three Knight classes, together with Lancer and Saber.

Rider: Servants placed within this class place emphasis upon speed and powerful Noble Phantasms, which often include their mounts. Their special skill is Riding, which allows them to fully utilize abilities of their mounts (which can range from simple horses to mechanical vehicles to divine or supernatural creatures.).

Caster: Servants placed within this class are adept in magecraft and have a special ability akin to Reality Marble, called Territory Creation, which alters or creates space around them to enhance their sorcery.

Assassin: Within the Fuyuki Holy Grail War, Servants placed within this class are always, as a rule, one of the 19 Hassan i Sabbah, the leaders of the clan of the Hashshashins who passed their name down. The Assassin in Fate/stay night summoned by Caster is an exception, an aberration in the system, as he was summoned by another Servant. This rule comes from how the word "assassin" itself finds its etymological roots in that clan. This rule is not present in the Moon Cell Holy Grail War and Great Holy Grail War, allowing for those known to be specialized killers to be summoned. The Assassins' special ability is Presence Concealment, which allows them to remain undetected. Offset by their stealth is their weak combat skills. Due to this, Assassins often target Masters instead of Servants.

Berserker: Servants placed within this class are always heroes who have gone berserk in their lifetime. This trait allows them to use the special ability Mad Enhancement, which trades their consciousness (i.e. sanity) for a large power boost. Most Masters are incapable of controlling their Servant once Mad Enhancement has been activated, which eventually results in their death.

A few side notes 
Noble Phantasms are basically a all or nothing attack since using one would take a lot of energy from a servant if used recklessly it can result in the death of the servant at the hands of a careful servant who managed to avoid/survive the noble phantasm even a master can manage to kill a servant who recklessly used one should only be used if it will ensure a killing of a servant like for example a saber using their noble phantasm to kill the wounded and unable to move archer would be a good use if the archer is otherwise able to defend themselves. 

Reality Marbles are a type of noble phantasm that can change the nearby landscape to a reality more suited to their purpose like a saber although unlikely that this would be their noble phantasm would create a world full of swords to pick up on the fly and stab at the enemy another thing about reality marbles is that they trap the target in it along with the user a reality marble can only be held for a few mins even if the servant is at full strength

Alright I believe you guys get the idea if you have any questions ask and I will answer them besides that the usual rules you see like no god modding etc. and have fun with your characters 

Saber's Master: Saber:Sakaki Chizuru
Archer's Master: Archer: Squall
Lancer's Master:ChunkyCaptain Lancer: ChunkyCaptain
Caster's Master:GuyYouWishYouDidntKnow Caster:GuyYouWishYouDidntKnow
Rider's Master:ZainWilder Rider:LaXnyd
Berserker's Master: Ishnorb Berserker: Ishnorb
Assassin's Master: Assassin:Gareth

Character sheet
Name:
Age/Age appearance:
Master or Servant:
Who is their Master/Servant:
Class (servant only):
Weapon(s):
Noble Phantasm:
Noble Phantasm description:
Magic (master/caster only):
Appearance:
Personality (can be left blank):
Bio (how did they become a magi? By blood or by extensive training? Who were they while they were alive before becoming a servant? What great deeds did they accomplish before they died?)

Name: Gilgamesh
Age: 26
Master or Servant: Servant
Who is their Master: No one
Class: None
Weapon(s): Many different legendary weapons
Noble Phantasm: Gate to Babylon
Noble Phantasm description: Creates a reality marble of sorts that has all of Gilgameshs weapons appear from a door that comes behind him to use as he sees fit unlike a reality marble it does not trap the target nor does it change the landscape in exchange for this flaw Gilgamesh can keep this reality marble up for as long as he wants since it barely takes any energy for him to use it
Appearance: [img=http://img2.wikia.nocookie.net/__cb20110807172201/typemoon/images/e/ea/GilgameshUC.png]
Personality:
Bio: Gilgamesh single handily created the greatest civilization known to man gathering many treasures and legendary weapons during his rule almost achieving immortality at one point before it was taken from him by the snake. To this day no civilization has ever come close to the greatness of his civilization which was achieved thanks to his greed and selfishness.

Name: Kain Alaster
Age 22
Master or Servant: Servant
Who is their Master: Open
Class: Archer
Weapon(s): Twin shortswords named Chrysaor and Sanglamore along with his bow named Gandiva
Noble Phantasm: Arrows of pain
Noble Phantasm description: Kain creates arrows out of his blood which was cursed by the maiden in black to create arrows capable of piercing through solid rock the more arrows he creates the more cuts that appear on his body making each new arrow stronger due to more sources of blood to draw from
Appearance: [img=http://i1088.photobucket.com/albums/i337/tres_hciiix/image623.png]
Personality:
Bio: Kain was the first Paladin and the first one to fall to the taint of the darkness in his quest to find a cure for the king of his kingdom he met the maiden in black who promised Kain a cure for his kingdom in return Kain had to vow to marry the maiden and to leave the kingdom once the king had fully recovered Kain agreed to this and was given the cure but as the king slowly recovered the less and less Kain wanted to leave his kingdom he had given so much to protect when the time came for Kain to leave with the Maiden he refused upon refusing the Maiden cursed his blood to be full of the taint that his king had been cured of and to be followed by death wherever he went a mere day passed after the maiden left when Kain found his king dead by the very thing he had just been cured of Kain knew the maiden had indeed cursed him so he left the kingdom and his paladin ways becoming a unholy knight Kains first task as a unholy knight was to find the maiden in black and kill her in revenge taking his bow and twin shortswords to do the deed Kain spent the next few years looking for her watching people die by the taint in his blood until the taint had finally started to take it's toll on Kain who later died from it never finding the maiden who had cursed him and the kingdom he had dearly loved. This is the second time he has been summoned into the holy grail war the first time was as Saber.
Archer's Stats
Strength: C-

Endurance: C

Agility: A-

Mana: D+

Luck: D

Magic Resistance: C