Ena is
Souma's younger sister, unlike him, she is very popular with the opposite sex and often brings men to her room. Her parents don't suspect her true nature and think she has a boyfriend but she makes them believe that "he is just a friend, nothing more". She already had, at least, 3 "boyfriends" (= sex friends).
She doesn't seem to care who can be her partner (before having sex with her brother).
When her brother interrupted her, she decided to take revenge by raping him. She immediately found their compatibility "excellent", telling him immediately that it was the first time she felt so good. Since, she becomes aware of her brother as a member of the opposite sex and tries to seduce him sexually.
(spoiler hidden)