Queen Guenevere
Guenevere was married to King Arthur and at first was as equally revered as he by his Knights. She sat with them at court and was present around his legendary Round Table. She was also a woman of great beauty, so much so that she was a temptation to men just by standing before them. Guenevere was a free spirit and felt constrained by the expectations of society that fell upon a queen.
Although Guenevere comes across in the poem as one who is not taking her predicament seriously, and also has an air of arrogance about her narrative, she is also incredibly frightened by the situtation that she finds herself in. By trying to minimize her actions she is trying to persuade the court that they should follow suit and look at her potential moving forward rather than at what she might or might not have done in the past. At the same time, she is clearly politically tone deaf and does not see that the writing is already on the wall for her before her defense of herself even starts. She does not understand the rivalry between Gawain and Lancelot and does not realize that her situation is due in large part to Gawain's exaggeration of its importance and his political pot-stirring amongst the other knights, many of whom envy Lancelot's close friendship with the king.
Guenevere is a beautiful woman although she is not at all humble about her beauty. She actually claims to be intoxicated by her own physical appearance which, although to her indicates that she is pure of spirit - her logic being, how can God have created anyone this beautiful if they were not beautiful on the inside as well - does not signify the same thing to the knights who believe that she has weaponized her prettiness and deliberately tempts them; Lancelot has succumbed to the temptation.
Gunevere really does seem to love Lancelot despite her protestations otherwise and this seems to be reciprcated as it is he who comes to her at the end of the poem when she needs him the most.
Lancelot
Lancelot barely features in the poem in his own right, but is a character much described by both Guenevere, in her defense, and also by other knights, which we know because Guenevere references it herself. Lancelot was Arthur's most trusted knight and perpetual wing man. He was valiant, brave, chivalrous, and on more than one occasion saved Arthur's life in both structured battle and against ambush from other kingdoms. There was huge rivalry between Lancelot and Gawain, with Lancelot being painted as the bad guy because of his close relationship with Guenevere.The relationship was very inappropriate but neither admits to its seriousness within the context of the poem. However, Lancelot, making an appearance in his own right at the end of the poem, dashes to Guenever's rescue, making himself the literal knight on a white horse rescuing the damsel in distress.
Gawain
Gawain seems to be the ring leader of the court. A man who has never liked Guenevere, chiefly because he overtures to her have been rebuffed, he is also very envious of Lancelot's position at court and of his close relationship to the king. Gawain has also saved Arthur from death on a number of occasions and there is some resentment that this has still not led to the kind of relationship that has grown between Lancelot and the king. Gawain's determination to punish Guenevere for her affair with Lancelot is more political than loyal to the king, and it is also not out of any sense of moral disapproval or prurience. He merely sees the demise of Guenevere as a politically and strategically expedient thing.
King Arthur
Although barely mentioned in the poem - which is ironic given that he is the figure at the center of events - Arthur is King and the husband of Guenevere, hurt and embarrassed by her affair with his most trusted knight. The majority of the knights of his court are extremely loyal to him and because of this see Guenevere as guilty before she opens her mouth to defend herself. As knights they have all shown a willingness to lay down their lives for him and so this is the attitude they bring towards protecting him from threats to the heart. Arthur is beloved and known to have been down-to-earth, asking nothing of his men that he is not prepared to do himself. This is another reason for their supreme loyalty.
