Possible, although I can't say that this is the case for everyone who suffers from DID.
When these alters take over, they often talk and act differently. We have seen Ray to be speaking the same language as his father to which I believe where Ray first emerged.
In some cases, one personality may also pick up certain habits that the other does not, such as becoming aggressive. People with DID are not dangerous or evil contrary to what is usually depicted in tv shows etc. Ray was 'violent' towards those who hurt him but he wasn't with Soohyun and the others. Just like what Ray said, he exist to protect. His role is to protect the host (Sooha) from any trauma that can possibly harm the host. Oftentimes, this kind of alter even takes the place so they experience the trauma instead. They try to manage feelings of anger and focuses most on defense. It is common for them to also have self-destructive behaviors, and can also be very aggressive verbally at times.
Things that might serve as a reminder of past trauma can trigger a dissociative episode such as the vase thrown at Sooha by his former doctor thus, causes Ray to appear.
An individual with DID may or may not be aware of the other personality states and memories of the times when an alter is dominant just as how Sooha was unable to remember the conversation Ray and Soohyun had and the clothes he changed into.
As for the act of transitions itself, it can often look like a small movement of the body or a sudden change in mood. The person may go from being anxious and dependent to being angry and assertive. Switching can be detected if paying very close attention and while being aware of the condition, but it’s very hard for casual observers to recognize one themselves. No dramatic switching such as having red eyes, however, I do think the author made it on purpose to show us the difference between the original host and the alter.
At the end, Ray, even though he thinks he's not needed anymore (now that they're safe), he is still uncertain whether he will be able to disappear and when. This is actually a good representation. Alters do not disappear completely as they are part of the person with DID. They either go dormant or integrates. Alters do stabilize over time (with prof. help) but since there is no integrated sense of self, whoever the brain recognizes the most fit to do the job for the system will be the one who is out most while the rest stays in dormancy.
Thank you! Now I can read this in peace. I’m actually a psych student (that’s why I don’t read much bl/manga in general that has mental disorders bcs some if not most were just written based on stereotypes) and based on your analysis I can tell that you’re really educated r u also inclined in psychology?
I'm glad my insights made you at ease. I'm in the medical field as well (I won't specify exactly what area) but unfortunately, not in psychology (far from it, tbh). I've always been interested in neuroscience as well as the human behavior, brain and mind hence, I involve myself in studies that are related to it as much as I can. Also, I wish you goodluck on your degree I'm not sure why my comments keep getting cut off mid sentence, sorry for spamming gaaaah. :<

In terms of the medical aspects of the story, is it based on facts? Esp the ml has a disorder, im very particular about these things bcs it irate me when authors write a story with a mental disorder but doesn’t study for it.