SATEN TWIST:
Although he usually means well. Saten Twist isn't always as innocent as the other my little pony characters. He's been known to kill 3 people (though non of them were on purpose).
Saten often reveals to have had a very bad childhood, his father was abusive to him and his mother, and his mother was emotionally distant, never showing him any true signs of love. As a result, Saten has grown into a recovering alcoholic, who sometimes takes weed, and has strong temper issues witch often makes him unpredictable, and sometimes even sadistic.
However, he is also shown to be quite immature and dosen't often take things too seriously. Cause of this, Saten is often considered one of the most sarcastic characters of the series.
Saten can sometimes be considered a bit creepy when it comes to woman, often flirting with them at the LEAST appropriate of times. And on rare occasions he actually stalks them before having gotten together with Trixie. Saten may not be winning any "boyfriend of the year" awards, but he would NEVER cheat on Trixie. And although Saten still flirts with most girls he meets, he never actually tries anything.
For the most part, Saten loves causing mischief on his spare time. And he tends to get along well with Pinkie, cause they both love pranking people (although Saten's pranks can arguably be considered a bit crueler).
But despite all his flaws and temper issues, Saten isn't "at heart" a bad person. He is strongly loyal towards the ones he cares about, especially Derpy, and later Trixie, not afraid of physically harming those who wrong them.
He is also quite nieve, as he never realized Trixie's true feelings despite how obvious it probably was. It wasn't till she told him herself, while revealing to be very heartbroken about Saten falling for AppleJack instead of her.. But Saten later made up for this and is currently her boyfriend..

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DERPY:
Aside from the cute and harmless appearance she gives off. This version of Derpy is your typical "tough girl". She isn't above using violence to get her way, and despite Saten's strong protectiveness, she very RARELY lets him fight her battles, she can show impressive fighting skills (she's once mentioned to have beaten up a stallion twice her size for picking on Lemon Hearts).
Saten's protectiveness of her has been known to sometimes annoy her, often telling him "I'm not that little filly anymore". But she understands that he means well, and still respects him for it in the end.
Like Saten, Derpy is a occasional Marijuana user, apparently she bakes it into her "special muffins".
Although Saten is older than her. Derpy is often more mature than Saten, sometimes parenting him. But this isn't ALWAYS the case, the roles often switch within the series. With sometimes Saten being the maturer one.
Derpy isn't above sarcasm, in fact, she is rarely shown NOT being sarcastic or sneer, often laughing at other people misfortunes, and joining Saten in most of his pranks.

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TRIXIE:
Trixie is more or less the same as the show.
But is often shown much more positively.
Her arrogance is only part of her stage acts (well, least after her redemption in MAGIC DUEL).
Trixie, despite her behavior in Boost Busters and Magic Duel, is actually a very kind hearted and loyal person, was just lonely (probably like the show).
She, Derpy and Saten are childhood friends. And despite when Saten threatened to kill her after hearing she lied in Boost Busters (he was drunk), she forgave him, and actually has an huge crush on him, but he was too naive to see it.. It wasn't till she told him herself, while revealing to be very heartbroken about Saten falling for AppleJack instead of her, but Saten later made up for this and is currently her boyfriend. Twilight and the main six trusts Trixie way more than in the show, as Trixie isn't as secretive..

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APPLEJACK:
Saten's ex girlfriend.
Whom he still had romantic feeling for (until he dates Trixie).
Although it took her many years to respect him as a friend, do to him being a negative influence on AppleBloom, and use to spy on her and once stole her t-shirt for unknown reasons.
But when they DID became friends, she shows to care a lot about him, and often likes his company, partially his humor.
Being one of the "normal" characters, much of her personality hasn't changed from the real AppleJack..

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MASTER SWORD:
As running joke in SATEN TWIST ADVENTURES, Master Sword often shows signs of being a bit "insane". Often saying and doing things that most would consider "abnormal" (like jumping out a 2 story window and hospitalizing himself simply to avoid being interviewed by Saten back when they first met.)
Easily described as one of the most foul mouthed characters of the series, Sword can sometimes come across as a uncaring jerk, but is actually a very kind hearted, loyal person, and very protective to those he cares for.
He and Saten have a rocky relationship, often arguing and humiliating each other. But they do care about each other. And often have more in common than they would usually admit to. Both are quite immature and love causing mischief on their spare time.
Saten Twist respects Sword enough to at least allow him to date Derpy. Who Saten is VERY protective of..

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STARLIGHT GLIMMER:
She is mostly the same, but not quite as lonely. She DID have a friend before NO SECOND PRANCES. Saten Twist and Derpy are those friends (and probably still Trixie, but she would of ALREADY knew Trixie).
Starlight, like Pinkie Pie and AppleBloom, has grown to see Saten as a bit of a big brother. And is very aware of his attraction to her (he makes it kinda obvious), but doesn't see him that way, and even if she did, she doesn't want him being "disloyal" to Trixie..

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RAINBOW DASH:
No info

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PINKIE:
Sees Saten as a big brother..

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TWIGHT:
Saten admits to be at least SOMEWHAT attracted to her. And their relationship seems to be mostly positive, though they do argue on occasions.

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RARITY:
They are often kinda mean to each other, but they DO care for each other in the end..

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APPLEBLOOM:
Saten is somewhat of an corrupting influence on AppleBloom, But she sees him as a second brother, and it's shown to be him that she's telling it to after getting her cutie mark. And she sometimes asks him for advice. Though he rarely ends up giving any.
AppleJack worries about AppleBloom's choice of having him as a role model. She knows Saten isn't exactly a good influence on her, but still lets AppleBloom make her own decisions (this being one of them).

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SCOOTALOO:
No info..

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SPIKE:
Despite often saying he hates Spike. Saten dosen't MEAN it. In fact, they seem to be very close..

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SWEETIE BELLE:
Sweetie Belle really likes him as a friend. Sometimes asking about for his advice, but like AppleBloom. he rarely ends up giving any..