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Levi has bipolar disorder, PTSD, and anxiety, also he has psychotic breakdowns.

Eren suffers from anxiety.

Levi is a guy who has a depressing past, so much so that he's been sent to a psych ward, for attempting suicide.

Eren who is also a patient at the psych ward, has to share a room with Levi.

Which annoys eren, because he is kept up most nights from Levi's psychotic breakdowns, and nurses have to come in and restrain him, from harming eren, or even himself.