This is Merlin in Season 1, his was just a young wizard who came to Camelot with big Hopes but as a wizard he couldn't use his magic but he will have to use it for save Camelot and the royal family. by saving Prince Arthur's life he is positioned as his Main servant. While this time Merlin and Prince Arthur were like enemies but as the series progresses that changes. During this time everyone thought that he had a crush on Gwen when they were just close friends, he was Morgana's friend as well. his was learning with Gaius not just medicine, he was learning about Magic.
This is Merlin in Season 5, Merlin has changed many things during this time, he has a new outfit and have grown a lot. his magic is more powerful than before. he has the same reckless personality but he is still kind a funny. Merlin have a strong friendship with Arthur, and he is always present during every mission. he is who cook the food for Arthur and knights. Merlin has experienced his first love and he make new friends. his No. 1 enemy is Morgana who now is a witch that he at the season end is killed by Merlin.
This is Arthur Pendragon in Season 1, during this time he was the son of King Uther so he was the Prince of Camelot. Arthur is good fighter and he always help to his father with the Kingdom of Camelot nesecities. he was an arrogant and rude prince who only lived to make pranks and jokes. As Merlin he is funny and when his father give to Merlin the chance to be his Main servant he was very upset with the idea because he hated Merlin but then he change his terms with him. he would do everything for Morgana who is like a sister for him . he always recurring to Gaius for a medicine and he treated to Gwen like any other servant on the castle.
This is Arthur Pendragon in Season 5, Many things have changed during this time, his friendship with Merlin is stronger and for him he is his only friend and the most he trust. his dad died and he rises as the new King of Camelot, He found out that Morgana is actually his half sister and as she betrayed Camelot he is now her enemy, he chooses new knights for Camelot and creates the round table. Since the first season he have been meeting many princesses for marry him but since season 2 he falls in love with Gwen and in this season they get married and live a few time with Gwen in the Castle. he loves Gwen with all of his heart and would give up his title as King just for her.
This is Morgana Pendragon in Season 1, she was a beautiful girl that was known as Lady Morgana. She was a kind heart girl who was always there for Arthur, Gwen and Merlin. she was the protected of the King. she was living the life of a Princess. she has a crush on Arthur and was always helping him. her best friend was Gwen that at the same time was her servant. she was against King Uther's rules. When Merlin entered to the Castle she knew that he was a close friend of Gwen and thought that he has a crush on her. she later found that those nightmares that she always was having actually were visions.
This is Morgana Pendragon in Season 5, Many things happened to her during the last years. Morgana found out that she had a half sister called Morgause who is a powerful witch, then she found out that she was the biological daughter of King Uther and the half sister of Arthur, so she was actually the princess of Camelot. Morgana change a lot her perosnality and the only thing that she wants is to kill King Arthur and to know who is Emrys (Who is actually Merlin) she is now a witch with super powers. she wants to take away Gewn's title and make Arhur to suffer. Morgana is more powerful than never. Her sister sacrificed herself for the sake of her sister. she betrays everyone in Camelot. she wants to be the queen of Camelot but a the season finale she was killed by Merlin.
This is Gwen in Season 1, Gwen was just a cute and kind servant of Lady Morgana who was also her best friend. Gwen lived with her father but he later die. she makes friends easy with Merlin for whom she falls in love but just for a short time. Then Gewn met Lancelot and he falls in love with her and her falls in love with him. Gwen during this time had a strong friendship with Merlin. She always recurred to Gaius when she were in problems, she hated the form how Prince Arthur treat to his servants. she was always with the hair upsweep.
This is Gwen in Season 5. Many things in her life have changed. Gwen started to have a relationship with Lancelot but Lancelot realize that Arthur was in love with her ( He falling in love with Gwen when he was in her house and stayed with her as a part of a task, there they share a kiss and started to have feelings for each other) so he decided to go away for the sake of everyone. She and Morgana weren't friends anymore. When King Uther die and Prince Arthur rises as the new King, he decides to ask Gwen if she wants to be his wife, she says yes but Lancelot (Who was dead) is coming to life by Morgana and Gwen started to have feelings for Lancelot so then Arthur notices this and Gwen is exiled form Camelot and Lancelot dies again. Days ago he found Gwen and as he love her with all of his heart he decided to forgive her and make her his wife changing her name to Gwen Pendragon and make her the new Queen of Camelot as Queen Gwen Pendragon.
This is Merlin many years later. By then Merlin is still alive and lives in the modern world as a very old Emrys . When season 5 ends Merlin killed Morgana but King Arthur dies too but first he shows his magic to Arthur before of his death then Merlin prepare him a traditional funeral. It assumed that the vidow Queen Gwen rules over Camelot until her death. In the modern time Merlin is waiting for the day to Arthur rise again. it is unknown if he is still using magic since the technology eases the lifestyle.