rider

 class: Rider
 real name: Medusa
 measurements: 172cm (height), 57kg (weight), 88cm, 56cm, 84cm
 colour: black.
 natural enemies: Saber, Souichirou.
 likes: reading (books of any genre), riding, drinking alcoholics, protecting Sakura
 dislikes: mirrors (logically), people who's against Sakura
 master: Matou Shinji (only temporarilly), Matou Sakura
 weapon: Her eyes, knives
standing: chaotic good
noble phantasms: Rider is peraphs the servant with most significant noble phantasms.
- Gorgon Breaker - Self-Seal . Temple of Darkness (as the legend says, Rider/Medusa can change people in stones, just by having them watching at her eyes).
- Bellerophon - Bridle of Chivalry (Rider can ride Pegasus, as Bellerophon did in mythology)
- Blood Fort Andromeda - Outer-Seal . Temple of Blood (A barrier of blood is casted around her enemies, adsorbing their mana and energies)

personality: Rider is a faithful servant, she's not that talkative as she prefers analyzing people (and testing them) than speaking about herself (and risking to unhide her identity or her abilites). She can't really stand Shinji, but she accept him as a Master because she has to. She's overprotective with Sakura, maybe because she's charmed by her kindness and her sweetness or probably because she thinks she can save her from a cruel destiny (it's like she can partially see the future and forecast events). She's also very logical and she doesn't act because of anger or instinct. She prefers to rely on her Master's abilites (tactical and magical) and on her judgement. Rider is a loner. She doesn't get along that well with people, mainly because she's a monster after all and she's used to be avoided and misjudged by people. She really misses the love of her sisters, although she realized they weren't acting all times for her own good, despite the fact they really were affectionated to her. She's not that evil as well, she's probably only egoistic (although we don't know her one true wish, but it's certainly something related to her unhuman condition) in her will to realize her expectations (but again all the characters could be considered under this point of view). She would like to look younger and cuter, as her sisters are.

spoilers warning!!!

personal history: Rider was summoned by Sakura, her real Master, although she initially appears as Shinji's servant. She doesn't really wish to collaborate with him, but she thinks it's also Sakura's will and she obeys to his commands. She helps him in setting the barrier at school, to gather mana from the students in order to summon Pegasus and use her Noble Phantasm in fights (and also because a large amount of energy is always useful in the Grail War). The reason behind this plans are quite clear: Rider isn't a so powerful servant and she really needs to be able to use her noble phantasms at full power in order to be able to compete and even beat her enemies. As Saber says, she's probably the one with the most powerful noble phantasms, although what she use is something only allowed to people who have many sins (the evocation of a creature like Pegasus). In the first scenario she's an immediate danger for Shirou, as she attacks him at school and although Shinji wants to make Emiya believe he's a friend, he doesn't really care about his mischievous actions. Saber is able to defeat Rider after a long fight in the school and later on the top of a building in the town centre. In Unlimited Blade Works Rider is defeated at an early stage and we don't really learn more about he; she is almost immediately eliminated by Souichirou (she's decapitated, how ironical!) She doesn't do that much, although she was a bit stronger than in Fate.
In the last scenario we discover a lot about her, her real identity, her reason to fight and her real potential. She's initially under Sakura's mastership and she fights following her orders, even when Sakura turns evil as she's commanded to guard Shirou and let him suvive. Thanks to Shirou she reaches her full potential in battle and she accomplishes her role as Servant as well. At the end of the war she decides to live with Sakura, until her mana will be finished and she'll fade away.
In Hollow Ataraxia she lives togheter with Sakura and she seems really happy. She even has a sort of crush for Shirou. Hollow Ataraxia let us know something more about her history and we're introduced to her sisters as well.
Medusa myth is somehow different from the classic one (that you can read in the next paragraph). It seems that Medusa was living togheter with Stheno and Euryale but she wasn't enjoying that life too much, although she really loved her sisters. They were both beautiful and immortal and Medusa, who was mortal instead, was treated like a sort of slave who had to attend to any annoying job and protect them and their domain. It happened that Medusa wasn't able to fulfill her duties and she was banned from their place.

the legend behind: In Greek mythology Medusa is one of the Gorgons, the only one who wasn't blessed by immortality. She was a beautiful woman, so pretty that Poseidon fell for her. He badly wanted to be with her, so he turned into a bird and he raped her in a temple sacred to Athena. The goddess got mad by those happenings and she decided to take revenge, especially because Medusa said that her hair were more beautiful than those of the divine maiden.
Athena turned her into an horrible monster with snakes at the place of her hair and with the ability to transform everyone into stones when directly looking at her glance.
Medusa is mainly famous for the episode if her falling, caused by Perseus. The young man was able to kill Medusa (and eventually her sisters, but it's somenthing not mentioned by all the authours) thanks to the help and the gifts of Athena and Hermes, who were helping him since the beginning of his quest.
Perseus was able to strike as the Gorgons were sleeping and he cut her head. Pegasus and Crisaores were generated from that letal cut, as well as other incredible things, like the two floods of blood that were able to kill or resuscitate people or the fact that the blood wich was flowing from the head turned itself into different things as it went in contact with different places (like snakes in the desert).
Medusa and the Gorgons could e frequently found in Greek mythology, so I may assume this is the most common and traditional interpretation of their myth.