More than you think **Spoilers**
This is not my original idea as i read a review making this theory, but this movie is more enjoyable when you realize it is not a forest that kills people. This is a forest where people must face their sadness, and some cannot face it and kill themselves. Now, onto the point about the main character...
Sara and Jess not twins but are one singular person in a split personality. There are TONS of clues to this.
1) The hair color is an obvious choice. It plays a role later, but to tell the difference, we see two dramatic changes. Could just be cinematic ease, however it does play a role later.
2) She always says that when her sister makes bad choices, she has to come and save her. Sara is the passive personality, and "comes forward" during times of high stress or trauma. This split originated during the trauma of the parents murder/ suicide. Sara said she never saw the scene, so when Jess faces that sadness it sparks Sara to the rescue as Sara can hide behind the idea that she never saw the trauma. One very specific case of this is the second suicide attempt. Sara said she had not heard from Jess and could not reach her on the phone, so she had 911 check on her. You call 911 locally. You do not call it across states, so Sara called 911 when Jess took the pills to save her. Another thing is the use of pills, which someone dealing with this level of scarring would have high amounts of medication, like the ones seen in her bag. If we were to look closely at the pills, I bet there would be heavy medications for schizophrenia.
3) When Sara tells Rob that she needs to go save Jess, Rob looks like he has dealt with this before. What he sees is that Jess has relapsed into Sara, and is doing his best to keep her grounded. By "sleeping on it," he is trying to buy time for Jess to take over, but in the morning she has all ready left. One small aspect is we never see her in a plane, only in an apartment and then in Japan. Jess was actually in Japan teaching when she had a snap, and Rob did not have to fly to catch up to her. It may even be possible that Rob is her psychiatrist, and the apartment was his office. More on him later.
4) When she dreams about Jess in a tent, you see two silhouettes, but when she opens only 1 person is in the tent, which is Jess.
5) Sara knew exactly which direction Jess' tent is in. We never see the two together ever, except in pictures or during the split event. Jess was lost for 5 days, yet when she runs to the rescue party at the end she was at her tent.
6) When Sara searches her apartment, she looks at all of the hair products (which I bet hair dye is one of them)
7) When she sees a picture of Jess on Aiden's phone, it was actually a picture of her he had taken for the article. She does not recognize it is her because she views herself as blonde, but Jess is the real person so any picture would be of Jess and not of her perception of blonde hair.
8) The Sara Teasdale poem
"And not one will know of the war, not one
Will care at last when it is done.
Not one would mind, neither bird nor tree,
If mankind perished utterly;
And Spring herself, when she woke at dawn,
Would scarcely know that we were gone."
Translation: No one knows about her splits warring for control, and no one will care about the Sara's death as she is not real. Neither bird or tree relates to the forest gaining another spirit upon the shedding of the sadness. When she awakes at dawn, she barely knows the war took place as she never acknowledged she was split, but she does scarcely know she is gone.
9) When she explains the twin link, its a sound or buzz, and when the other is dead, it is silent. That is the quiet in her head after the split no longer occupies her subconsciousness. In this case, both personalities have a reality, but only one exists. The noise is the presence of the other personality in her subconscious.
10) When Jess comes out of the forest, Rob says "Sara?" They have opposite hair, so he should recognize that it is Jess. When she says nothing, he changes the name to Jess. She then answers. To further this, he strongly asks her if she has seen Sara. Knowing Sara is the trauma, he needs to know if Jess has regained control. When she says she is still in the forest, Rob knows it may mean Sara has been eliminated from Jess. She also says "she came to save me," which I think is when she realizes that Sara took control. More than likely, Jess understands their is a Sara, but is unable to control when she takes over.
So, to sum up the story. Jess moves to Japan and takes on a job as a teacher, but has been dealing with this mental health and trauma from the child's view of the horror. She knows the forest is where people "find themselves" and can "face their sadness." She had a tent, brought along reading material as she is their for a walkabout to finally face her trauma. During this, Sara takes over while she is in the forest. Two things could happen here. Since the story is told from Sara's view, most of the Sara adventure is just a completely false reality happening while Jess is in the woods, or Sara completely takes over, changes clothes (why Jess' clothes are hanging in the woods) and goes back to her psychiatrist in which she believes she is married to. Then the story goes as we follow Sara through backtracking to Jess. Jess said she was lost for 5 days, meaning she was in the woods for 5 days, but that is also how long we have followed Sara.
The story comes to an end in the ranger's cabin when she kills Aiden, and then is faced with the basement scene. She protects Jess from seeing the scene, telling her to not look, but then Sara sees the scene. Sara would not know what this looks like, as she said she never saw it but the scene was obvious to her. This is where Sara is forced to face the fact that she saw the horrible scene, and cannot hide between a split that is a witness to it and one that is safe from it. She then finally, once and for all, faces it, by literally asking her dad why, and then cutting him away from her. This of course is the suicide of Sara, but it is the killing of the split, leaving Sara to die in the basement where she was created. After the suicide, Jess awakens, and both can be seen running parallel to each other, indicating the return of control by Jess. This is the moment where they pass each other in the main control of Jess. Jess comes forward as Sara is running, causing Jess to run. She eventually emerges from the forest, having faced her trauma, and left behind a soul for the forest to keep. The Yuri are the remnants of the sadness left in the forest by the people who faced their sadness, regardless of suicide or success.
Nichi could not handle the sadness of losing someone under his guide, and this is why he is now susceptible to seeing the spirits, and Sara comes for him in the end.