How Matou Shinji came to be
Nasu: Shirou's intolerable friend. But he's not just an intolerable guy. He's a mistaken troublemaker with a distorted past. And we thought we needed a human-type friend character who devotes himself to his desires and does whatever he wants, like "if he became a Master, he'd abuse that power, right?". So Matou Shinji was born.
Takeuchi: In the sense that he's a normal person, this is a character who's in a familiar position to the readers.
Nasu: Though he's also a character with that sort of personality (laugh). Shinji doesn't have the ability or qualities of a magus, and has plenty of ability for a normal person. If he'd had someone by his side willing to be his sycophant, he'd be a man who gives off a pretty good impression.
Takeuchi: Like kind of an annoying Holmes. But there wasn't actually any sycophant, and Sakura was around causing his inferiority complex, so his heart became twisted.
Nasu: It became so that he couldn't sustain his self-worth except by scorning others. If he hadn't had any talents, he might not have gotten so warped. Just because of those talents, the awareness that "I'm better than other people" built up more and more. But beside him was Sakura who had the ability as a magus that he could never reach no matter how hard he tried, and in the Matou family that was a lineage of magi, he was treated as though he didn't exist because he didn't have that ability. So looking down on the people around him was the only way of life left for Shinji.
Takeuchi: I have no room to sympathize, but he had a real reason for being selfish...? Even Shin-chan has his pride.
Nasu: But the Matou house would have been more fun if Sakura treated him like dirt! We were talking about Gilgamesh earlier, and honestly, if someone like that became your Servant, most Masters would want out of there. You'd think "hell no, I ain't teaming up with this guy", right. But Shinji is just happy about it. "I'm the Master of someone this strong? Then I'm going to win."
Takeuchi: He's great, in a sense. In my case, if Gilgamesh appeared and said something like "use me as you wish", I'd immediately answer "please go away". Anyone would (laugh).
Nasu: But you could call that a strong point of oblivious Shinji, thinking of himself as equal with Gilgamesh who's unique even among Servants. Even though he's wrong, being able to fearlessly command Gilgamesh is a talent.
About Matou Shinji's design
Nasu: What I requested was simply the image of "the most handsome guy in school and a bit of a wannabe-stylish type".
Takeuchi: I took the order, and thought up a normal good-looking guy with creepy facial expressions, but he looked like Gilgamesh with his hair down, so it was changed to the current wakame hair.
Nasu: You could have given him a bit more of an unpleasant handsomeness though.
Takeuchi: It's strange. That's what I was going for, but eventually I noticed that rather than creepy handsome he became more of a madao (utterly good-for-nothing old guy) instead (laugh).
Shirou's friend Matou Shinji
Nasu: At school, Shinji's grades are always in the top five. His grades are good and he's the eldest son of a family with a mansion in Miyama, so it feels like he's always got girls hanging onto him. They probably wouldn't hang out with him in university, but during the high school period he's the perfect subject to squee over. Plus he has money.
Takeuchi: So it was about the money?! Ah, before and after the Grail War, did Shinji's mental state regarding Sakura or Shirou change?
Nasu: Because he was called "useless" just for the body he was born with, becoming a Master was the best thing that ever happened to him. Leaving Sakura aside, maybe he seriously wanted to team up with Shirou. When Shinji says "I'll use you", it means he appreciates you. On the other hand, when Sakura summoned Rider, he was sulking like "I guess the time has come". But when he saw that Sakura didn't want to, he took a chance with nothing to lose and said to Zouken "leave it to me, not someone with no motivation" and the response was unexpectedly "all right". A huge turnabout for Shinji. Still, when Shinji who wasn't a magus said to Rider "I'm your Master from now on" he ended up getting rejected. So he had Sakura make the "book of the false attendant", and became able to command the strong Servant Rider. From then on his mind was finally at ease. Then he felt more lenient, like he could forgive a few things. Because of this, in "stay night", he became twice as unpleasant as the usual Shinji. Actually, the normal Shinji is not so violent (laugh).
Takeuchi: He felt lenient, but he got worse? (laugh) Just how full of himself is Shinji?
Nasu: That's inevitable. Anyone would become arrogant if Rider became their Servant! That's all kinds of Heaven Condition. By the way, to Shinji Shirou is a rival, and at the same time he's the only man who will associate with him without ulterior motives. Even while calling Shirou an idiot, mentally he has a special view of him unrelated to like or dislike. You could say Shinji wants Shirou by his side, he's irreplaceable. It's unknown whether or not Shinji himself realizes it, but he looks at Shirou and thinks deep down "it must be nice to be able to live like that...". But in the story we needed a "easy-to-understand villain", so he became an ostentatiously bad character. I should have jotted down some line in the story like "he's an unpleasant guy, but he shines too".
Takeuchi: Shinji only survived in the Rin route where he messed around the most. Maybe it's more like a reward for trying so hard. It seems like he became better after that, right?
Nasu: Maybe because he became like "the Grail War is over, so I don't have to worry anymore". And he saw his own limits, and in particular Rin risked her life to save him (laugh).
Takeuchi: He grew up, huh. Someday the day will come when you too look back fondly on these days as a dark history... By the way, was his relationship with Sakura always like that?
Nasu: No, when Sakura was just someone who was adopted by the Matou family, he loved her even as he picked on her somewhat. But the moment he learned that the successor to the family was not him, but the outsider Sakura, the positions reversed. Like "the one who was feeling pity for the way the other was born wasn't me, but her?" At that time, he lashed out with feelings of shame and disgrace so much that he wanted to die.
Takeuchi: I feel a refined madao power in the way Shinji unconcernedly uses violence against women.
Nasu: Shinji is habitually violent against Sakura, so he's not opposed to using violence against women. Isn't it scary what people can get used to? So, like I said before, the reason Shinji is violent to Sakura is because he always subconsciously thinks "if I don't control her, then some random person is going to usurp the Matou family". Before he knew she was the successor, he picked on Sakura but still loved her somewhat, but once he learned the truth he was seized with the obsessive idea that "if I don't completely repress her, there's no place for me" and resorted to violence.
What you wanted to convey through the character Matou Shinji
Takeuchi: His position is an essential character in a romance that takes place in a school. His origin was that kind of "necessary for the story", but his unique flavour must be the reason he became a long-hated, and then beloved character.
Nasu: I already said this, but he's a normal person who attained great power and ran wild. In a sense, he is the character in the closest position to the reader. I wanted to convey that. But it's not "true". In the world of Fate, "true evil" refers to people like Kotomine who were broken from the start.