Riser was the main antagonist in Vol. 2. Issei hated this man with a passion at first because he was "stealing" Rias away from the group, which Issei did not want. At the end of the Rating Game between Riser and Rias' peerages to determine whether Rias should wed Riser, Issei went against Riser but ran out of stamina and was beaten to near-death before being saved by Rias, who surrendered in order to prevent any further harm to him. He woke up three days later and went after her, interrupting the engagement party, and Sirzechs decided on a one-on-one battle. During that fight, Issei gave up his left arm to reach his Balance Breaker and ended up defeating Riser and taking Rias. Issei made him go into a depression due to the fact that he has never lost and also made him afraid of Dragons. Later on during the invasion of the Underworld, he overcomes his fear of Dragons due to Tannin's training and went out fighting and also tried to encourage Ravel about Issei's death. Riser didn't really hate him after he took back Rias.
Rias was previously engaged with Riser. However, Rias was against this and decided to use a Rating Game under her parents' suggestions to nullify her engagement with him. When Issei was about to be killed, she begged him not to do so and said that she will marry him.
Ravel is Riser's younger sister. Not much is shown of their relationship at first, but as said by Isabella, Ravel is their Bishop, for him to have a little sister type and tsundere in his peerage. Riser is somewhat protective of her, stating to Issei that if he ever mistreats her, he will burn him to a crisp. He also cares about her, with him and his older brother, appearing in Volume 12, to cheer her up.
In contrast to Riser, the rest of the Phenex clan are composed of well mannered and humble people. Thus, the family did not hold a grudge on Issei, for defeating Riser, as his victory over Riser had made him go into a heavy slump; on the contrary, they were thankful for the Issei for doing the deed as the family found it a good experience for Riser in order to fix his arrogant and unruly behavoir.