Jean Grey has the power to take over the world. She could mind control everyone, and those who couldn’t be taken over mentally would be taken down with her telekinesis. Despite knowing that, Jean manages to be a gentle soul. She believes that being non-violent is the way to go. There have been several occasions where she’s tried to use her words rather than overpower someone. It doesn’t always work, but she’s always willing to try.
There’s also how she is with the X-Men. Many of them look at her as a sister or a mother. She’s always looked for the good in people. It’s one of the reasons that Wolverine didn’t leave the team early on. Even though he’s attracted to her, he loved that she sees things in him that no one else does. It helped him continue to be a better person.
How does this connect to Eleven?
After everything that the scientists at Hawkins Lab did to her, no one would be shocked if she became evil. Yet, she became a hero. Eleven managed to come out of all that abuse and torture and remain kindhearted. We can assume it’s because she didn’t want to do what was done to her. Coming in contact with her friends definitely helped that. The first people she meets are good to her. Leaving Eleven to realize that not everyone on Earth is terrible.