Not much is known about Eri's relationship with her parents but it seems as though they loved her, but after she accidentally made her father disappear into nothing with her Quirk, her mother was so distraught that she disowned her. Eri's mother left her in the care of her grandfather, the boss of Shie Hassaikai, wanting nothing to do with a daughter she saw as "cursed". Her 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 Shie Hassaikai, 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. Since the rescue, Izuku's earnest and genuine desire to see her happy again made her grow fond of Izuku, seeing him as an older-brother figure.
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. Soon after they become closer with Eri seeing him as an older-brotherly figure.
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. He also seems to be teaching Eri about various holidays and customs. Due to Eri's severe psychological trauma and physical torture at the hands of Overhaul, Shota has been working to rehabilitate Eri while she remains under his guardianship. To this end, he utilizes the extremely positive social influences of Izuku and Mirio to help her gain better control of her emotions.
In time, he aims to teach her how to control her Quirk so that she may be able to restore Mirio's Permeation. He is very protective of her, not only out of concern for her well-being but for the safety of those around her.
Overall, Shota is very competent in his role as Eri's foster parent and guardian, doing his best to ensure that her physical, psychological, and developmental needs are met.
After being taught by Mirio that Neito is U.A.'s "dark side", Eri becomes frightful of him, and she cowers at the sight of him when he arrives to copy her quirk.
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.