Ravel did not think much of Issei when they first met, but she started to admire him after he defeated her brother Riser to get Rias back. She was shown to have developed feelings for him by Volume 5, and have fallen in love with him by Volume 12, cemented by her feelings of grief and sadness over his 'death', and her happiness at his return.
It is implied in Volume 9 that she enrolls in Kuoh Academy so she can get closer to Issei, and moves in with him in the following volume. She happily accepts Sirzechs's request of becoming Issei's manager and continually works hard for him, such as finding a suitable magician for him to make pact with.
Ravel explains at the end of Volume 14 that the reason why she grew to admire and later fall in love with Issei was because of stories her servant used to tell her as a child. These stories were about heroes who sacrificed or suffered greatly in order to protect people and the ones they love. As she grew, she gradually forgot these stories but recalled them after seeing Issei getting hurt, beaten, and eventually winning in order to save Rias. She then began to research more about him, learning that despite his perverseness, he cares about his friends and family and how much he resembled the heroes that her servant used to tell her as bedtime stories. It then grew to the point where she wanted to get closer to him, to her fully realized hero and idol, and support him as the person she respects and developed feelings for.
Ravel and Koneko have a hot and cold relationship due to the fact that she is a Phoenix and Koneko is a Nekomata, alluding to the natural rivalry between birds and cats. They see each other as rivals because both are in love with Issei and will openly compete with each other for his affections. However, they have been shown to get along and have become friendlier to each other as time goes on, usually only showing hostility to one another when it involves Issei, such as in Volume 14, where they are shown to be fighting over the spot on Issei's lap.
Ravel is Lady Phenex's only daughter among her four children, as well as being the youngest. Lady Phenex is quite aware of Ravel's love for Issei and her desire to be useful to him in any way. She has also traded Ravel from her son, Riser's peerage for an unused Bishop piece, and has gone on to tell Issei that Ravel is currently a "free" Bishop.
Ruval is one of Ravel's three older brothers, he being the oldest of the three. He is seen with Riser trying to cheer up Ravel after she had learned Issei had "died". He then leaves to fight the monsters created by the Annihilation Maker.
Riser is one of Ravel's three older bothers, he being the youngest of the three. Originally, Ravel was part of Riser's peerage before she was traded to her mother's peerage for an unused Bishop piece, though it seemed that she herself did not take part much in Riser's Rating Games. The reason Riser added Ravel to his peerage (likewise his harem) was because he had wanted an imouto (little sister) type girl, as well as a tsundere type girl.