Not much is known about Eri's relationship with her parents, but after she accidentally made her father disappear into nothing with her Quirk, her mother repudiated her, saying she was cursed, leaving her to her grandfather, the boss of Shie Hassaikai. His grandfather took care of her, and asked his subordinate Kai Chisaki to find out what her Quirk was. Kai discovered that Eri's power was the frightening ability to Rewind, and developed a plan to use her Quirk to develop a Quirk-Destroying Drug that would restore the Yakuza to power. However, the Boss refused to consent to Kai's inhuman plan, so Kai induced him to a comatose state.
Izuku is the first person to ever touch Eri with kindness. When she tries to escape from the Eight Expendables, he refuses to turn her over to Kai, forcing her to run back to him when she thinks Kai would kill Izuku and Mirio Togata. Afterwards, Eri recalls Izuku reaching out to her as "different" and "kind".
During her rescue, Izuku is the first person not to call her Quirk a curse but rather kind and gentle, driving Eri to tears of joy.
She first meets Mirio at the same time she meets Izuku, although they do not react well. When Mirio finds out about what has been happening to Eri, he blames himself greatly and becomes determined to save her. He is the first person to give her hope that she could be free of Kai and the Shie Hassaikai.
He later invites her to the school festival and holds her in the air for her to see Izuku perform.
Eri was unable to control her Quirk after Izuku defeated Overhaul, causing Izuku visible pain. Shota, albeit injured, was brought to the scene so he could erase Eri's out-of-control Quirk. Since then, Shota has been somewhat of a guardian for Eri, sometimes taking charge of her care as she lives with the U.A. faculty in the dorms. Eri is comfortable with Shota's presence and listens to him well.
Shota appears to have a soft spot for Eri, having bought her a matching set of cat-themed clothes while she was in the hospital for her to wear when she was discharged.
Eri is terrified of Kai. Similar to how he treats his members, Kai only values Eri because of her use, this being her Quirk that aides Kai in creating a world with no Quirks. Due to his own Quirk, Overhaul, Kai does not hesitate to endanger Eri's life, as he can simply fix her if he acts soon enough. In addition to using Eri as a lab rat, Kai mentally abuses her. He tells her that she and her quirk are a curse and causes problems for everyone. As a result of this Eri developed a habit of grinning and bearing it to Kai's research efforts believing there was nothing else for her. She consequently loses her ability to smile.
After her rescue from Shie Hassaikai Eri still wasn't able to smile. The memories of Kai's torture leave her doubtful and afraid in her new life. However, with the care U.A staff and students she is able to smile and become happy at the U.A. School Festival Arc showing she is recovering from Kai's past influences.