hiii so there were a few people who didn't understand (i totally get that because i didn't either tbh)

so here is an explanation:

ellie actually has bipolar disorder and multi personality disorder

bipolar disorder means that she can be very happy and lovely but her mood can switch in a second, changing her into a violent woman.

multi personality disorder is a very rare illness, so basically it's just that a person can be multiple people, and he or she changes between those personalities: for example, i'm sarah, a lovely girl with a nice heart, but suddenly my personality changes and i become alex, a girl who idk likes to kill people. alex nor sarah know who the other personality is so the person/body doesn't know she's changing her personality all the time. 

so basically she's kind of crazy (i'm not saying people with a disorder are crazy so please don't judge me) because she has two really really difficult disorders and her sister commits suicide because of ellie. harry doesn't know anything about ellie's disorders though, until she attacks him. her dad stops her an takes harry to the hospital were he recovers, but he still wants answers from ellie... but you'll never get to see those haha, just make something up.

and after attacking harry, ellie's dad sends her to a mental institution (where harry is in the epilogue)

if you don't get it here are some wikipedia facts: 

bipolar disorder is a mental illness characterized by periods of elevated mood and periods of depression.[1][2] The elevated mood is significant and is known asmania or hypomania depending on the severity. During mania an individual feels or acts abnormally happy, energetic, or irritable.[1]They often make poorly thought out decisions with little regard to the consequences. The need for sleep is usually reduced.[2] During periods of depression there may be crying, poor eye contact with others, and a negative outlook on life. (source: wikipedia) 

Dissociative identity disorder (DID), previously known as multiple personality disorder (MPD),[1] is a mental disorder on the dissociative spectrum characterized by at least two distinct and relatively enduring identities or dissociated personality states that alternately control a person's behavior, and is accompanied by memory impairment for important information not explained by ordinary forgetfulness. These symptoms are not accounted for by substance abuse, seizures, other medical conditions, nor by imaginative play in children.[2]

i hope you now understand a little bit more! (if you don't just comment!)