Considering how well the characters are presented in the game I will attempt to not focus on aspects consistently present in the game.
(Good) His sense of justice most likely comes about as a result of a reaction from the injustice common among peasants and the common people of a royal city, and so being given a position of power most likely allows him to indulge in the feeling of being able to make a major difference and impact in the live's of his subjects. Arthur seems like the kind of king that if no one reminded him that he needed to maintain the security of his allies, and needed a castle to provide allies with a good impression then he would probably break the castle down either selling it by piece or giving them away to citizens who needed it then use the bare necessities to provide a living place for his court and himself. His willingness to accommodate the requests of potential allies and hear others out provides him an openness to every possible opportunity to forge an alliance with anyone who even considers it. Every other virtue of his is consistently clear including kindness, optimism, excellent leadership and providing for the good of the people.
(Bad) Much of what is considered good about him is also a very serious flaw. Aside from qualities his enemies have already taken advantage of in story it is also a flaw that he tends to be easily influenced and his optimism could eventually suffer a serious blow especially if he realizes he can never bring about peace mostly because his subjects will never be satisfied. It is also possible that Arthur places so much importance in his vision and his ideals because they are an aspect he has consistent control over, and therefore attempts to extend into action as a reaction of having been told that as king he would loose much of his independence because of royal responsibilities.
(Good) He is a charismatic individual, most likely due to the awareness of how a variety individuals are very distinct from one another and trying to maintain good grace whenever possible thus he has a remarkable attunement to his emotions and passions. There also his skills in the direct application of warfare. While Arthur specializes in organization and the theory of what should win a battle Lancelot has more knowledge when it comes to being in the midst of battle, spy networks and methods that contribute to being prepared for whatever the enemy has to throw at them which demonstrates a need to be in the middle of the action and adventurous as possible to fulfill the dream he always had before having to handle his responsibilities. Yet it is clear that his loyalty is stronger than the force or armies.
(Bad) He doesn't handle new or unfamiliar territory well. After being used to a lifestyle in which he would commonly flirt with ladies and seek their affection his behavior changes quite dramatically and becomes a bit uncharacteristic according to those close to him most likely due to the sudden forced infatuation between Guen and himself. In addition to this his desire for adventure seems to have also left a sense of dissatisfaction within him despite doing his best to be in the middle of the action and seek all available adventures. Despite the fact that he feels keeping certain secrets is for the good of anyone involved, if taken to the extreme his skills and practice could present potential for him to be a remarkably effective liar and deceiver.
Morgana (Personal Favorite)
(Good) She is the most studious, focused and determined to do good for those she cares for. Although she is motivated by an intense interests in her studies in dark magic, and if possible would devote all her time and energy to that much in the same way that Merlin does she is happy to be helping Arthur and Lancelot out of a begrudging respect for both of them deriving from their talents and vision. Despite her slight to mild annoyance and/or distaste towards men(I'm still trying to figure out how severe this prejudice is, if any since her occasional remarks may simply be out of occasional frustration.) she is no radical feminist and is willing to work with them and develop at the very least a friendly relationship with them.
(Bad) Her work also creates a sense of isolation and distance towards most people. It seems to inspire more impatience towards others, and not demonstrate as much empathy with most individuals. Thus she is not exactly the person you want to rely on to win over a potential ally at a fun party that requires a decent amount of charisma. There are also many varieties of intense emotions she keeps bottled up inside her, not feeling comfortable in sharing or discussing certain feelings even with the likes of Arthur and Lancelot who are very close to her. Causing an intense amount of inner turmoil she seems only comfortable handling with someone who is very close to her and her interests. Considering she engages in intimate intercourse with a Guen romantically inclined to her on a crew occupied boat despite the clear risks.