Sir Kay
Sir Kay was Arthur's foster brother, and is often depicted as have a fiery temper. Read more here and here and here.
Sir Bors
Sir Bors is most well-known for accompanying Sir Galahad and Sir Percival on the Grail Quest. Read more about him here and here
Sir Bedivere
Sir Bedivere's biggest role in Arthurian legend is as one of the very few survivors of the Battle of Camlann against Mordred and his forces. Bedivere returns Excalibur to the Lady of the Lake and brings a dying Arthur to the isle of Avalon. Read more here and here
Sir Gareth
As brother of Sir Gawain, Sir Gareth is often over-shadowed and over-looked. One of his tales is here, although it is almost certainly entirely fictional. Here is another version, and here is the original by Tennyson.