Haven't thought of a confirmed beginning, but here is something I have as of right now.
The main character is a student who lives in a room with three girls and two guys who are silent and talk to him often. To find out what is going on with them, he decides to follow them group one night since they often are gone at night. After following them to a dark place (like an abandoned buikding or something), he finds the group as Yangs fighting a powerful Yin. He intervenes and kills the Yin, who has overpowered the group and nearly kills one of girls. Though he saves them, they are also angry for him following them and killing the Yin. However, they are forced to tell them what is going on and the protagonist hopes to join them in fighting the Yins.
Just for some character development so far for the main group.
- Yang of Seriousness: More recognized as the leader of the group due to how she takes things seriously when needed. She becomes frustrated when the protagonist learned of their activities, but becomes more concerned him to avoid him becoming an Yin. When she does takes things seriously, he gets enhanced physical and mental abilities, including increased strength, speed, mental toughness, and flexibility.
- Yang of Wicca: Though not the leader, she is the most powerful one of the group. She has a calm, mature personality. Her main abilities are in relation to Wicca, using powers of fire, water, earth, air, and spirit. She can also create circles and create spells and charms to heal and benefit her allies. Her main responsibilities are medical treatment and sealing defeated Yins.
- Yang of Robotics: The smartest of the group. She is naturally intelligent and has a great interest in robotics and engineering. Her main abilities are to create robots out scrap metal, using the designs she makes. These help the group a lot from providing support to searching for enemies and traps.
- Yang of Delinquency: The toughest of the group. He had a rough and poor childhood until he was found by and joined the main group. He has abnormal strength and very good at close combat, but is still short of his leader.