"FOOLS! My mom is NOT a traitor! She's on a secret mission for the King! And now I must KILL you, because YOU KNOW TOO MUCH!"
Shinji is reaching that awkward stage of childhood. He is full of defensive resentment over his mother, and pretends that she's on a mission for the king in deep cover. Or that she was framed. Or that evil Queen Mutsumi has cast a spell on everyone. Or, after a talk with Setsuna, that Cone People are to blame.
Shinji likes to exaggerate his own abilities. He constantly claims to be a great master of Pattern like his brother and sister, having snuck down and walked the Pattern by himself. Shinji actually has about as much Pattern as the average herring. He draws trumps that don't work, and blames it on 'Mutsumi jamming his cards'. He believes that he has a potent mind, a skillful swordarm, and a tactical mind surpassed by none. Sadly, none of these things are even remotely true.
When feeling bitter and unhappy, which is much of the time, Shinji likes to concoct elaborate schemes to get back at whoever he feels is responsible for his mother's predicament today. They invariably go pear-shaped, because while Shinji is very bright and has an active imagination, he is not a very good tactical thinker and tends to incorporate powers or skills he doesn't actually possess into them. Then he gets frustrated and tends to berserk people, because he IS very strong and a good martial artist. For his age, that is.
Shinji hero-worships but secretly resents his mother, and hero-worships and not-that-secretly resents Nagisa, who is raising him. This is more a result of his current 'phase' than anything else. He idolizes Kaname (who taught him to love naval combat), Chloe (who taught him that Pattern is the coolest thing ever), Nanami Minor (who taught him to beat people up), and Shiori (who wears tight shirts), all of whom have spent a lot of time with him and been kind and understanding.
Shinji's Staff of Power was a birthday present from his sister Chloe, a fairly simple Pattern artifact. Shinji is tremendously proud of it and pretends that it surpasses spikards and Patternblades in potency. He also uses it to fake his 'Pattern Master' routine. Not very well. His Galleon is a present from Aunt Kaname, and has been responsible for many, many groundings and one diplomatic incident involving the conquest of a Therinese fishing village.
Shinji has learned to shapeshift because none of his relatives can. His demonform looks a lot like the old paintings of Shinobu Major, only with horns, fangs, scales, a tail, etc. He's scared of her. And of Drake, whose beard is also incorporated into the form.
Despite being a raving delusional emotional muddle, Shinji is actually as nice a kid as you'd expect someone raised by Nagisa to be, and will likely become a pleasant young man once he pulls out of puberity and gets more of a grip on his feelings, which is likely to happen by his mid-teens.
Despite coming from one of the most hyper and noisy parents in all of reality, Rei is strangely quiet and is more often than not content to just watch events happening around her. She does smile, laugh, cry, etc. but infrequently enough to make people wonder about her. Even when being hugged rabidly by her hug-happy clan, she is oddly passive. She will also hum miscellaneous songs on occasion, but has never tried to sing. She is known to scribble on her brother Nemo's face with crayon when he sleeps.
She is a water elementalist of surprising skill, something that pleases Random and greatly worries Jacqui. In rare moments when Rei throws a temper tantrum, she tends to abuse her abilities badly. She also sometimes bites when angry, though her bite is not nearly as bad as her mother's.
Rei likes Coca-Cola.