When we first met to create my players' characters, the first thing I asked was to create normal characters (e.g. no freaks/depression/obsession/psychologically disturbed, etc). Normal people with normal lives, career events permitting (it's a Traveller game). I do not like freaks and I believe that you do not need a freak to make an interesting and fun character to play. (I believe a more experienced player might manage to roleplay a freak character in an interesting and not problematic way.)
One player, after making him rewrite his character twice (he was making freak characters), created a very interesting one (finally!). He told me he likes freak characters, but only after we started talking about the freak characters he was creating, and not before. He did not understand how a normal character could be interesting. I tried to explain, but apparently he did not understand my point.
Now this player is very inexperienced. I'm convinced that he plays like this because he has no experience but hack&slash. By now he seems to have grasped my point that normal characters can be interesting to play now, because he's not roleplaying a freak. However, he simply does not roleplay as the character he made. So, how can I help him to roleplay the character he made?
TL;DR: My inexperienced player described his character one way, but he's roleplaying it completely different. Since I think it's due to his inexperience, how can I help him roleplay the character he made?
Some Background
The first time he created an apathetic medic without emotions who saw people as machines to be repaired. That's not normal, that's a psychological problem! In a realistic setting (as is hard sci-fi) no one would take him as a crewman, in fact probably they would take a person like this to an asylum.
When I say I made him rewrite the background I mean I told him something like “This part is not ok because it's a freak thing. Change it please.” I absolutely did not write his background as I wanted it, I only told what and why a certain thing would not be ok with me, because looked too freakish to me. Of course I made suggestions for his background, but I believe that's part of a GM's “job”.
His character is a medic-born person that happened to become a pirate, started to despise pirates and pirate life, and decided to become a Marine (as a medic) to fight them and bad people.
The problem is that he's not roleplaying his character at all as he has described it or how it's supposed to behave: as a medic (most of all) and a marine. I realize that saying that he's supposed to roleplay it in a certain ways is bad – it's his character, after all – but how he behaves is probably not what the character he described would do.
He's inexperienced in RPGs, and in fact this is the first roleplaying campaign (strictly speaking) he's ever played in. This is probably the cause, and it's not his fault, but my problem is that when other people try to explain to him why his character would not behave in a certain way, he seems refractory to our explanations and gives us illogical justifications. He doesn't even seems to follow his main goal (the search for his missing love).
I tried to ask him if he wanted to change his character a bit, but he said he does not see any reason to and continued to explain to us why he behaves like that. I fear he will continue to roleplay a “freak”, or that he'll change the background for the worse.
A more experienced player of mine suggested changing his character by some incident in game and a roll of the dice. Something like “you have been wounded in the head, now you behave strangely,” in order to make it more similar to what he seems he would like to play, but that seems to me like cheating and a cheap solution.
I didn't know the Same-Page Tool before we started to play, and maybe using it could have helped, but I think the main problem is that he's just green at roleplaying and can't “enter” into the mind of his character.
Campaign info
This is a sandbox-style game. It's also in a hard sci-fi setting, where people should behave and react as in real life.
I am the GM, and this is my second GM experience (not counting few one-shots) and I've never been a player myself (sadly).
I told my players "give your PCs a motivation to travel in space and a reason of why they're together".
The PCs' names are Vincent, Lucien and Ryan. Lucien is my problematic player's character. The main goal for now is to find Vincent's father, who left him with his mother when he was a child, and sister (who is Lucien's love). Before the campaign started, Lucien saved the life of Vincent's sister, and fell in love with her. Due to his Marine duties he could not stay with her and she disappeared again. Later Vincent found information about him and decided to meet him to get more information. Lucien had no further information, but they became friends. Vincent then decided to get a ship and to take Lucien and Ryan (a common Marine friend) as crew and investigate about his sister.
So, why can't he be a freak? Because sooner of later (before finding his sister) Vincent would see how Lucien really is and would dump him in space before letting his sister stay with a freak (even if they do not know if she loves Lucien). And, being a freak, I'm not sure he could have managed to enter the Marines and stay there so many years.
The players know about each others' backgrounds, we all worked them out together.