Rider was Shinji's Servant now. Now that he had the book and the seals, she was bound by his orders instead of the one who had first summoned her, Sakura. That didn't mean she had to like him. But that was irrelevant to the Grail War in any case. She wanted her Master to win so that she could win. The fact that he was a miserable worm wouldn't have bothered her at all, if it weren't for Sakura.
The way Sakura effaced herself and deferred to Shinji had always unnerved Rider. The jealousy behind his treatment of her was transparent. The fool couldn't see that it only made him look weaker. Rider tried to ignore it. It was bad for morale, to say the least. The more time she spent around him, the more she enjoyed the thought of losing the war, just to revel in his failure and pitiful excuses. It could have stopped there, just a sick fantasy, if he'd behaved himself.
"Rider! Don't you EVER interrupt us again!" The force of Shinji's shout was quickly undermined by a panicked glance to the side. Rider didn't need to look at the book lying on the nearby dresser to feel the hold of a command seal.
"Understood, Master." Before she turned to leave, she wondered at the fear she'd seen in Sakura's eyes, looking up from the bed. Again, he'd said. This was probably nothing new. Sakura was probably less afraid of his usual treatment than she was of having anyone else find out. But now that he'd wasted a command seal in haste, he was certain to give all his anger and blame to Sakura.
Well, a command was a command. There was nothing she could do for Sakura right now. But while she couldn't interrupt... surely Shinji would be too preoccupied to notice her brief absence while she went shopping.
The funny thing was, Rider had no trouble fitting into this corner of the modern world. The girl at the counter welcomed her as an obvious connoisseur, had no problem with her inquiries, and quickly found just the thing she needed. She helpfully explained everything and even complimented Rider's outfit profusely, in particular her "kinky blindfold."
Rider quickly returned to the mansion, found a suitable place to hide the evidence, and said nothing to Shinji about the incident. He said nothing either, taking great pains to avoid her hidden gaze. The next day, he was suddenly determined to step up their plans with the Blood Fort.
She still fought at her best, the challenge of facing Saber giving her renewed purpose. But even after saving him and pulling off a desperate retreat, Shinji only came back with more abuse.
"Rider! What kind of shit Servant are you?!" Shinji paced forcefully through his study. "All that mana you stole and you still can't beat Saber? You still can't beat EMIYA?" He punctuated his sentences with the occasional kick at a doorframe, or a corner of the sofa. "We'll just go somewhere else, gather even MORE mana!"
"Master, I'd recommend more caution for now. I spent a great deal of mana just on our escape." Rider sat still on the sofa, her soft words enraging him further.
"Exactly! You made us run away with our tails between our legs!"
"If we strike again so quickly, Emiya will surely come after us along with Rin. But you saw their argument before. Their alliance could crumble if he truly believes we've withdrawn."
"Of course, let Emiya feel sorry for me... that's just brilliant! Oh wait! He already does! Because I'm the loser with the crappiest Servant!"
"Surely you know at least one more way for a Servant to gain mana." Rider's voice went cold, and Shinji stopped his pacing.
"Oh ho...?" Shinji's smirk was suddenly back. "That's the last suggestion I would have expected from you, Rider."
"Well consider it, Master. I'll meet you in your room shortly to discuss your decision." She vanished as she dematerialized back into spirit form.
Shinji sat down for a moment and chuckled softly. What an opportunity. He would have surely come up with the idea himself sooner or later, mana or no mana. But the way she suddenly offered it herself made him build up to hysterical laughter. He'd have to remind her who was the Master around here. As he walked down the hall, he could feel his grin stretching from ear to ear till it hurt. He entered his room and heard the rustle of clothing from inside the bathroom. How surprisingly shy of her. This was going to be even more fun than he first thought.
"Well Rider, you've convinced me after all," Shinji said, going right for the door and turning the knob. What he saw made him freeze, even though he certainly wasn't looking at her eyes. He tried to back away, but she grabbed hold of his arm and violently twisted it behind his back. His bravado crumpled into a babbling mess of whining half-words as she pushed him over the bed and tugged at his clothing. Desperation forced him to pull together some coherence at the last moment.
"RIDER!! WHAT DO YOU THINK YOU'RE DOING?!"
"Careful, Master," she interjected, "You wouldn't want to use up your very last command seal, would you?" Rider asked sweetly, leaning over his back to whisper in his ear.
"I... Damn it, you can't be se-!" his last protest was cut off by a high-pitched squeal. Rider laughed - he was every bit as amusing as she'd imagined. Rider didn't know if this would make things better or worse for Sakura, but she knew she couldn't follow meekly in her former Master's footsteps. And seeing Shinji grovel like this gave her even more pleasure than any wish the Grail could have granted. Maybe this false pretense could even serve to restore her mana after all...
She'd have to thank that lovely girl at the store for telling her about "pegging."