I dont won't to go into too much detail to avoid spoilers :)
but he has the symptoms of split personality, also split personality is often a result of childhood trauma.
Also there's a quote from MedicalTrialToday:
"he condition represents someone who struggles to integrate and
assimilate certain aspects of their own identity, which become
disjointed over time."
Also about the Symptoms fromsame article:
- Experiencing two or more separate personalities, each with their own self-identity and perceptions.
- A notable change in a person’s sense of self.
- Frequent gaps in memory and personal history, which are not due to normal forgetfulness, including loss of memories, and forgetting
This perfectly captures Reiner's behaviour. He acts almost like he has 2 separate collection of memories - sometimes he acts like a regular Scout who doesn't remember himself having a mission to capture Eren.
The only factor against the split personality is the fact that Reiner can access all his memories like both of his personalities become one again.
Also, if you're not familiar with manga or season 4, here's a very minor SPOILER:
Reiner also suffers later from depression and has suicidal attempt (he puts gun into his mouth), which is something that some of patients with DID suffers from too). -
There's also other explanation for his split personality - titan shifters can have memories and personality trades of shifters that had the same titan power before them. But i dont believe it's the case for reiner. END OF SPOILER