Though I don’t really like the writing about Eurus in the show, I like the character. So here are some ideas, every illustration has its own story (or maybe could related to others).
Eurus is very smart, talent, and close to her little brother, who understand each other the most.
2.Potterlock. Which Eurus became an Obscurial.
3.Mirror. Eurus = dark!Sherlock. Maybe not really ‘dark’, just the other side. Past and memories.