Due to Marinette's experiences in life, between her father leaving at a young age, to feeling abandoned by Eren as a teenager. This girl has a lot of mental illnesses that include hallucinations that make her see and hear things that aren't there.

Visual hallucinations

Visual hallucinations involve seeing things that aren't there. The hallucinations may be of objects, visual patterns, people, or lights.

For example, you might see a person who's not in the room or flashing lights that no one else can see.

Auditory hallucinations

Auditory hallucinations are among the most common type of hallucination. You might hear someone speaking to you or telling you to do certain things. The voice may be angry, neutral, or warm.

Other examples of this type of hallucination include hearing sounds, like someone walking in the attic or repeated clicking or tapping noises.

Mostly the hallucinations are a combo of Auditory and Visual hallucinations. It starts off as visual, and since she's in the Scouts, she has these visions of people that have died, such as one of her closest friends Marco who's body she had seen at the mission in the Trost district. It's not long after that the auditory ones start and she hears the screams of the people she heard die. And some, tell her that if she had been good enough that people would still be there. This causes her to have a low self esteem and believing that things are her fault even if she would have no way of doing so.

Mari also has a type of schizophrenia, although at the time they are set they don't know what kind she has yet, she has a type that she mostly hides from people so that they don't have to worry constantly.

Paranoid Schizophrenia:

Paranoid schizophrenia is characterized by predominantly positive symptoms of schizophrenia, including delusions and hallucinations. These debilitating symptoms blur the line between what is real and what isn't, making it difficult for the person to lead a typical life.

Symptoms
Early symptoms of schizophrenia may seem rather ordinary and could be explained by a number of other factors. This includes socializing less often with friends, trouble sleeping, irritability, or a drop in grades.3  During the onset of schizophrenia — otherwise known as the prodromal phase — negative symptoms mount. These negative symptoms might include an increasing lack of motivation, decreasing inability to pay attention, or social isolation.4

Warning signs that psychosis may be imminent include:5

Seeing, hearing, or tasting things that others do not.
Suspiciousness and a general fear of others' intentions.
Persistent, unusual thoughts or beliefs.
Difficulty thinking clearly.
Withdrawing from family or friends.
A significant decline in self-care.
Displaying all these symptoms doesn't necessarily indicate the presence of schizophrenia, but these are indications that a mental health evaluation is advised. If the person is experiencing the onset of schizophrenia, early intervention is the best chance of a positive outcome. 6

The positive symptoms of schizophrenia—things like hallucinations and delusions—are less likely to go unnoticed. After the prodromal phase, the patient enters the active phase of schizophrenia, during which they experience debilitating thoughts and perceptual distortions. They may experience impaired motor or cognitive functions, including disorganized speech and disorganized or catatonic behavior.

The paranoia in paranoid schizophrenia stems from delusions—firmly held beliefs that persist despite evidence to the contrary — and hallucinations — seeing or hearing things that others do not. Both of these experiences can be persecutory or threatening in nature. A patient may hear a voice or voices in their head that they do not recognize as their own thoughts or internal voice. These voices can be demeaning or hostile, driving a person to do things they would not do otherwise.

Odd, untypical behavior flows as a result of these delusions and hallucinations. Someone with schizophrenia may be convinced that the government is surveilling them in an attempt to harm them in some way. This can lead to boarding up their home, blacking out windows, putting objects in front of doors to impede entry, and otherwise blocking or removing items they believe contain listening devices or cameras. They may stay up late at night to catch culprits.

Someone with active paranoid schizophrenia is consumed by their delusions or hallucinations. The vast majority of their energy and attention is focused on keeping to and protecting their falsely held beliefs or perceptual distortions.7

The most common time a person seeks initial treatment for schizophrenia is during the active phase, when psychosis often makes a dramatic disruption in one's life and the lives of those around them.

After the active phase, the patient enters the residual phase of schizophrenia. Much like the residual subtype, hallucinations and delusions attenuate at this point (usually with the help of antipsychotic medication and other forms of treatment), and the patient experiences primarily negative symptoms.

Autophobia is the fear of being alone or of loneliness. Solitude, even in a safe place like home, can cause extreme anxiety in those who struggle with the disorder. Even if it is clear that they are physically safe, patients still may fear: Strangers or intruders.