sunny_marie_
This isn't everything the book has on them, I picked out any personality and physical descriptions, some relationships, and some things I thought might be good for plot as well.
I also added my own thoughts as well.

Gareth
Youngest son of Morgawse, brother of Gwaine, Gaheris, and Agravaine.
-I was thinking maybe they don't know who their mother is, and also Gareth knows that they're siblings but Gwaine doesn't, yet at least (and we can just leave the other two out (for now at least) since neither of us took them)
Physically well built, a great fighter, handsome, and courteous. Knows how to please the ladies by singing and dancing.
He arrived at the court anonymously and requested only food and drink for twelve months, in an attempt to discern who will befriend him and which knights are worthy of his fellowship and admiration. He was ridiculed and nicknamed Beaumains (don't know what that means) by Kay.
After twelve months as a kitchen knave, goes on a quest where he bests Kay and is knighted by Lancelot, creating a strong bond between the two.
-It says he loves Lancelot above all others his entire life, which really just makes me think he wants more than just friendship between the two which could cause some interesting problems maybe?

Kay
-There are two Kays but the other only has two lines so I'm giving you the one that picks on Gareth if that's alright.
Son of Ector.
Sometimes depicted having superhuman qualities and sometimes only a mediocre knight.
Tried to take credit for pulling the sword from the stone
A mocking spirit who gives unflattering nicknames to Gareth and Breunor le Noire.

Palamides
Son of king Astlabor, brother of Safere and Segwarides.
A follower of the Questing Beast, a beast that supposedly has a serpent's head, leopard's body, lion's backside, a hart's feet, and is capable of making a noise similar to that of sixty baying hounds.
I don't know what it means to be a follower of the Questing Beast so you could ignore it, come up with something, or we could also search it.
Rides with Tristram to the feast of Pentecost, where the grail appears.
I have no other information on that except I know no one drinks from it.
Defends himself twice during the tournament of Surluse against charges of treason.

Percival
Son of king Pellinore or Alain.
Brother of Lamorak, Dornar, Aglovale, and Alein. Half-brother to Sir Tor. Nephew to queen of the Waste Lands.
One of the knights on the quest for the holy grail.
-I believe there were three. Percival, Galahad, and one other. Galahad was the only one who was offered a drink from the grail, which is part of the reason why he's known for being the pure knight.
Him and Galahad are both known for staying celibate, Percival because he wants to be worthy of the grail and Galahad might just be waiting till he's married or just isn't interested at all, it doesn't go into detail with him.
Reportedly keeps fellowship with a lion
Goes to Sarras with Galahad but does not share in the final vision.
Doesn't linger in court long.
-It says nothing about his appearance but I will say I'm used to Percival being big and muscly because of the show Merlin.

Tristram
-There was no Tristan in the book but there is a Tristram so I'll give you that one and you can decided wether or not to keep him.
Son of Elizabeth and Meliodas. His mother died in childbirth, his father calling him a murderer. His name means "the sorrowful born child." Nephew of king Mark of Cornwall, cousin of Sir Andred and Alisander le Orphelin.
A lord of great lands in his own right and stays away from court as much as possible.
One of the finest examples of the traditional heroic virtues of courage, prowess, and loyalty.
Saves his step-mother, the daughter of king Howel of Brittany, from being burned at the stake for trying to poison him.
His father sent him away to France with his squire, Gouvernail, where he learns to harp, hawk, and hunt, and he was a good musician.
Returns to Cornwall until eighteen.
Slays Marhaus and keeps his sword and shield.
While he performed great feats of arms in Cornwall and Ireland, he wants to duplicate his success in Logres in order to prove himself up to the standards of the Round Table.
Fights Blamoee de Ganis for king Anguish of Ireland, wins, and spares his life.
Gets along well with and is loved by all the other knights.
There is more about him but it's all weird and bad and doesn't fit with our rp, it would also ruin how amazing he seems.

There is also stuff about how some of them die in the stories if you wanna know that.
I know I listed a few places here, but we're gonna have our own fantasy setting just cause that's what we already have, so we could just make up names to replace them along the way.

There's also a lot of mention of Guinevere's private dinner, where poisoned apples kill Sir Patrise and she is charged with treason. I was thinking maybe that happens, but instead Lancelot claims he did it in order to save Guinevere (who was framed by someone, we can come up with who later when we know who their enemies are) and that's how he ends up getting banished.