If you haven't played Mystic Messenger completely, you might read something that will reveal to you the complete story of the game. So please pay attention!
Being the twin brother of Seven, Saeran's hair and eyes color were originally same as his.
He dyed it white (With pink tips) at some point. At the end of Secret 02, it appears to have reverted to the same shade of red as Seven's.
There is also a possibility that Saeran's white hair is due to a syndrome called Marie Antoinette Syndrome which one's hair rapidly turns white due to extreme amount of stress, shocking news, fits of rage and extreme fear and anxiety. The tip of his hair the is the original colour of his hair.
His first appearance has him having mint colored eyes. At the end of Secrets 02, his eyes remain the same. It is implied that he probably wears contacts or it was a side-effect of the elixir used by Mint Eye.
During the prologue, when he asks the player if they would help him return the phone and the player replies with, "No. You're creepy.", no matter what the player chooses to reply with to his next message, he will proceed to send you an image of a complete stranger, which is intended to mislead the player into believing his words. (Though it's possible that the person is himself because it has been hinted that he wears contacts, or the image is found from the internet.)
It is implied in a story mode on day 10 Yoosung's route, that Saeran killed his mother.
However in Ray's Route Good End, it was Rika who killed the mother.
In 707's After Ending, Unknown - under his real name, Saeran - stated that he has no interest in women; however, this is only stated in Seven's dream. There is neither proof nor confirmation of what his sexuality is.
In the second Secret Ending, it's told that Saeran has a stress-related gastritis and a mild mental disorder, although it's not specified what kind. His immunity system is also low, presumably from the different drugs he was given.
It's also heavily implied that he suffers from depression, clearly stating that he wants to die and even tries to kill himself before being stopped by Saeyoung.
It is also stated that he was sickly as a child, which may relate to the low immunity he is noted to have when he's hospitalized in Secret 02.
While not official, Saeran's voice actor interprets Saeran as having Borderline Personality Disorder (which he himself has), and mentions it in his Free Talk for Secret 02.
According to the RFA Top Secret Book:
His email is firstname.lastname@example.org
He holds a special talent for hacking.
He likes ice cream and candy, and dislikes bread.
His most used app is the GPS.
His favorite quote is, "Perseverance is key."
He has broken the fourth wall:
In Jaehee's route Bad Relationship Ending 1, he says "Don't be surprised. I only came here because you didn't seem to want to play this game any further."
However, it is also possible that his was referring to "talking to the RFA" as a game, since he is revealed to have a tendency to speak in gaming analogies.
In April fools DLC 19:47 "Banned plaster bust", he says "This app is called Mystic Messenger."
In the games, Saeran has been stated to be good with wires, which is how he was able to access the window of Rika's 14th floor apartment.
In the game, there is a time inconsistency of when Rika kidnapped Saeran. In the SM after the chat room "We are on our way" on Day 10, it is stated that Saeran was kidnapped five years prior to the game. However, in the SM after the chat room "Our Plan" on Day, it is stated that Saeran was kidnapped and was already being drugged and brainwashed over eight years ago.
Ray is currently studying the language of flowers.
Ray considers flowers to be his only friends.
Of all the main characters, Saeran is the only one with two different greeting icons on the game's homepage. One for Ray, and a masked one for Unknown.
V also has two different greeting icons, albeit less obvious. One without sunglasses, and one with sunglasses.