Let's assume Issei is an idiot and trusted Tamamo from chapter 1. Now let's see how Rias deals with. Rampaging, heartbroken Issei into her lair of deception!
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In another timeline in which Issei did believe Tamamo-no-Mae and becomes her vessel, the fox-dragon spirit devil enters the scene! And he goes directly at the source of his anger!
"Rias Gremory...!"
"ISSEI, NO...!"
And shouts were heard inside the Gremory mansion as the doors leading to Rias' study room were burst open. Emerging from the darkness, the red dragon emperor came, rage being emitted from his right crimson eyes and steam oozing from his Boosted Gear! By the other side, protecting herself with a magical barrier stood his former King, Rias Gremory, eyes dripping with tears, not of fear, but desperation and sadness for having her dear dragon turned against her in such a way.
Issei, however, didn't buy it: after Tamamo opened his eyes regarding the scummy, deceitful devil that was Rias Gremory, his broken heart was lost, darkened, filled with the void of the truth, the bitter truth that his beloved Rias has been manipulating him this entire time. And not just him: Akeno, Kiba, Koneko, Gasper and even the likes of Sona and Sirzechs were dancing on the palm of her hand, laughing out loud as they all played like fiddles into her fingers. But not anymore, oh no, the perverted puppet died in that moment, and what came forth was something greater, something Rias could never expect: the Shinto dragon, the one equal to heavens, the one and only mate of Tamamo-no-Mae!
With a new master, and gaining some boost in powers for some reason because apparently being sad and miserable makes you stronger instead of depressed and unmotivated, he had a new objective: destroy the house of Gremory and all the vestiges of the chains that has been manipulating and destroying so many lives and factions for sick amusement. Done and done, the next day Issei bursted into the Gremory mansion, causing havoc and injuring several servants of Gremory, including the likes of Zeoticus and Venelana, making them all pay for deceiving someone like him. Room after room, door after door, the red dragon finally reached the one room where Rias was, and once inside, he prepared to not only bring her down, but revel in her misery and desperation as every thing she has and earned would be crushed under his newfound power, and he would enjoy every second of it! Revenge, indeed, was sweet...
"Why are you doing this, Issei?! What that wretched fox have turned you into?! Please, snap out of it!" She cries as her flimsy barrier was completely shattered by the red dragon emperor, and he stood tall against her, ready to figuratively and literally stomp on her! "I love you, Issei! Please, don't do it!"
"You deceived me, you deceived us, for the last time, devil." He coldly declared, holding the redhead devil by a hand and making her seat on a broken chair as she sobbed and cried for her dear Issei, turned mad after his encounter with the evil fox Tamamo-no-Mae, and now, the ultimate Shinto dragon was off for some old-styled reveageance. And yes, this is a real word, apparently. Aiming an arm at her, he declares as his palm flashes a dangerous red. "But before I obliterate you alongside this wretched cursed place, I want you to know this: I know everything."
"W-What? What do you mean, Issei?!" Rias screamed, her face glittering with tears as she reaches her hands for her dear Issei, even his acts of violence not being able to smear her love for him. In response, the dragon simply pushed her back to the broken chair.
"Don't play dumb with me, you cursed devil! Tamamo told me everything: she told me about your plans, your wretched ways, your ways to manipulate others and make them your puppets! First Akeno, then Koneko, then Kiba, Gasper, Asia and me!" Roared the dragon, his arm growing more ominous and dangerous as if he was indeed ready to destroy not only the redhead, but the entire Gremory state. By the glow of such, he was serious! Yet Rias could only cry in fear and doubt, not knowing what do in that situation! "This ends now! I'll destroy you together with this hellish place!"
"Issei, please don't! I love you!" She screams.
"Liar! I know that you made me die on that day with Yuuma! You knew from the start I was going to die by her hands, and yet you let me just so you could appear on the last moment as my savior and make me become your loyal dog! Just like you did with everyone!"
"How could you think that, Issei?! I just knew fallen angels were invading my property in Kuoh, that's why I knew about her beforehand to look out for! But I never would've thought she would kill you!"
*Ziiiim~...!* That... Was the sound of power going down.
When Rias could finally open her eyes, she saw that the glow coming from the Boosted Gear has ceased, and from Issei's face, there was no more rage, but pure confusion. If it wasn't threatening her a minute before, she would've thought it was cute.
"Wait, what?" Inquired the red dragon. He wasn't trying to humor her last pleas, he was genuinely confused about her sudden outburst, but still on the edge regarding her intentions. Rias, seeing the previous calm state of her beloved, continued.
"Yes, I knew about fallen angels into my territory, that's why I asked my peerage to keep an eye on them so they wouldn't get out of line!." She said, dryin her tears and trying to recompose herself, or at least look dignified, even against the rage of her beloved red dragon. "But it was all begrudgingly. Back then, the ceasefire between Fallen angels, heavens and the Underworld was shaky at best, so I didn't make a move on them not only because it would compromise the peace treaty between the three factions, but they were on their territory on abandoned churches and masses! Only when they did try to make the first move and revealed to be doing their own bidding was then I knew I had to act! I don't know if you notice, but I'm far from omniscient!"
"And how can I trust you?! For what I know, you could be lying yet again!" Issei roared once more, but feeling a tint of regret when he saw Rias moan in fear. She was... Too good in playing innocent, Tamamo has warned him as such!
"I'm not lying! Remember?! I even had you scolded for acting on your own and saving Asia!" She continued. "Don't you remember? You took the pamphlet with my rune in it from one of my familiars, that's why I came when you asked to have your wish fulfilled and, seeing your potential, reincarnated you into one of my pieces! Sure, I knew you might have a thing of interest, but what makes you think I wouldn't just ask you otherwise, knowing you were a nice pervert?!"
"I... I...!" Issei was starting to lose steam, and his mind filling with doubts everywhere. Could it be... No, Tamamo opened his eyes! Rias was not to be trusted, she was only a well of lies and sins, and nothing else! "Don't think you can convince me with those flimsy excuses! You just wanted to play savior so I could dance around your fingers! You let me die by Yuuma's hands!"
"Issei, please think clearly! That fallen angel was supposedly working under lord Azazel! If I was as horrible and manipulative and apparently all-knowing as you think I am, I wouldn't even bother saving you considering Azazel ordered to kill you! I would assume you would be either useless or too dangerous to even someone as powerful and influential as lord Azazel would want dead! If he supposedly wanted you dead because you were deemed too dangerous, why would I ever think of trying to persuade you?!"
"W-Well, when you put it like that... Stop lying, you sow!" Issei kicked another table and cursed Rias so he wouldn't be deceived once again. "Like I would believe you! How about Akeno?! Kiba?! Koneko?! Gasper?! Asia?! How is even possible for you to be so lucky to get all those powerful servants?! Isn't that a bit contrived?! I know you must've played your cards to get all of them into your peerage!"
"Issei, please don't be stupid!" Rias continued to cry. What was going on?! That wasn't her issei, and it was breaking her heart to bang heads with him! "Yuuto was a failed experiment of the Holy Sword project, a research under the Grigori which was later revealed to be an independent group as well! I just took him in because he was rejected and ready to die, otherwise if I tried to 'save' him beforehand, I would put the shaky peace tread between the Grigori and the underworld into jeopardy! Same with Akeno: she was the daughter of a fallen angel and only after she rejected her ancestry I took her in before her own relatives could kill her! And Koneko was marked to die before my own father stepped in and offered her to me, knowing full well she was sisters with a (previously thought) murderer who could also kill me at any time! And how could've I manipulated Gasper into thinking I was his savior when that was exactly what happened?! You think I would hire vampire hunters to track him down so I could 'save' him or somehow make his family mistreat and alienate him?! He was already ostracized and abandoned by his own kin, he would think I was his saviour regardless if He was half-dead by the time I met him! And I stepped in to save Asia AFTER I discovered it wouldn't compromise the Underworld's relationship with the Grigori because of that fallen angel's actions!"
"Shut up! Shut up!" Issei chanted. "So what?! You want me believe that it was all a contrived coincidence?! Laughable! As if I would believe that all those special someones would just happen to be at your disposal by the time I met you!"
"Then why don't you doubt the Grigori or the heavens as well?!" Rias inquired, a bit more composed despite that tears were still streaming from her eyes. "The Grigori has Vali, Tobio, Ms. Lavinia and all those other Longinus users! And the heavens have Griselda, Mr. Dulio, Irina, Mr. Strada all even other Sacred Gear users! You telling me its perfectly plausible for them to have their own one-men armies, but when it happens to me, something is fishy?! Also, if I were to look for servants just for their abilities, don't you think they would already have unlocked their powers by the time I made you my pawn?! Don't you think it's strange that I didn't force Akeno to use her Holy lightning, or Kiba to get his balance breaker, or Koneko to train and use her senjutsu or Gasper to use his Sacred Gear or even participate in my first Rating Game against Riser if I were only interested in their abilities?!"
"Uh... Oh shit... Uh...! Whatever!" He lifts his hand once again, ready to strike! "That doesn't excuse the fact that you're a spoiled devil princess! You look pretty and composed, but you're rotten and lazy to the core! Doesn't even try to train or become stronger when it's convenient for you, just like that time with the Rating Game against Riser! If you really wanted to break the engagement, then you should've trained yourself and your peerage instead of being a lazy bitch!"
"Please don't call me a bitch! That's hurtful..." She whines, a bit hurt. "And the rage and deception is clouding your mind! It was clearly stated that my engagement was to be settled by the time I finish university! Riser just did what he wanted to decided to try to marry me right away! Please don't think I didn't wish to become stronger, or that I just train my dear pieces when its convenient! You saw what we were able to accomplish in just 10 days, I knew you could grow stronger! Also, I don't know if you noticed, but thinking that I don't want to have a strong peerage while at the same time claiming I chose my pieces solely on strength, abilities and lineage is pretty contradictory!"
"Uh... Well... How about you?! If you wish to be the champion of the Rating Games, why don't you train yourself?! Why do you force us to train day and night when you just sit there and do nothing?!"
"Please tell me you jest or just don't know the rules of chess or Rating Games." Rias moaned. "Issei, I'm a king, the lynchpin of a peerage! If I fall, my Evil pieces fall behind regardless of their power! Even lord Azazel thought I engaged too much into battle in a Rating Game when a king should just let their pieces do the work, and I try to contain myself even then! You can't win with just a King, whatever be chess, a Rating Game or even in life! I trust my peerage's strength, I trust your strength, that's why I know I can let you, my dear evil pieces, do your jobs instead of engaging completely and risking losing a game prematurely! I might be strong, but I have to also strategize and think with my pieces for the best outcome, otherwise might as well have a target painted on my head!"
"..."
"..."
"What about...?!"
"If you're gonna say 'You are a spoiled princess whose brother and father do everything for by abusing their power', spare me of thinking you're less intelligent than I thought. Brother Sirzechs does love me a lot, he even helped break free from the engagement deal, but under his own volition, I was ready to marry Riser otherwise because I lost. And in case you forgot, my biggest wish is to step away from my brother's or my family's shadow; this is why I went to Kuoh in the first place in case you forgot that as well: to be known as Rias and not just Gremory."
"..."
"Issei... Think about it. Please... Even if you everything you said is true and I am a manipulative sow like you assume I am... What are you going to say to the others? 'Don't worry, that bitch that gave you shelter, food, protection, a place to train and rest and fake love that might as well be genuine won't hurt you anymore'? 'Hey Yuuto, I bet you wanna have died with your friends than being the knight of that whore Rias and having the opportunity to train with one of the greatest swordsman in history'? 'Akeno, did you know that your supposed best friend was manipulating you this entire time? I bet you wanted to be on the run up to this day fearing for your life'? 'Koneko, your life would be so much better with your sister who CLEARLY stated she wanted to kill you during your first reencounter and who abandoned you for no reason'? 'Gasper, I know being killed by hunter sucks, but hey, it's better than be under the protection of that cunt Rias who never forced you to do anything you don't like, even letting you skip a rating game in which your powers would definitely have helped and break her out of her engagement'!"
"Uh...?"
"And finally... 'Oh boy, I can't believe I trusted that bitch Rias who spoils and pampers me just so she can manipulate me and in no way have deep, completely strong feelings for me after all I did for her! I wish I was dead, killed by my own fake girlfriend under the (supposed) orders of a fallen angel man who clearly I trust with all of my heart since apparently I can put my faith on a man who EXPLICITLY STATED would have me killed if I even stepped out of line or even hinted to be a threat, but trusting a hoe known for loving her servants like family based on speculations, misunderstandings and assumptions is stupid and retarded. I rather trust this complete new stranger who came out of nowhere and talked me out of her control because I'm so smart that I will listen to him/her once and have all my previous knowledge shattered just because s/he said so, regardless... Of how many times Rias said she loved me and cared for me... And thought she could... Trust her servants to love and care for her back... Chuif, chuif..."
"..."
"Hic, hic... Aaaah~..."
"So... Uh... You're not... Manipulating me?"
"No! Of course not! I love you, Issei!"
"Neither does anyone?"
"Of course not!"
"..."
"..."
"Wow... I thought I was smart by putting those pieces together... But I think it's quite the opposite."
"Yeah, that's called gaslighting, Issei Hyoudou." Vali said. You might be wondering why he is here: turns out, with Issei's disappearance and the commotion he caused in the Gremory, the Lucifer was naturally drawn to the scenario, and inadvertently saw everything between his rival and the redhead, only seeing how he would reaction. (Un)fortunately, Issei calmed down and no further fight would occur. Issei turned to his rival, and sighed. "My father had that happened to him too. Fortunately you broke free before further damage could be done. Good for you, although you should've known better."
"Y-Yeah, my bad." The red dragon scratches the back of his head. He turns to Rias, who was still drying her tears, and bows. "Sorry, Rias."
"..."
"..."
"It's okay. I guess that happens..."
"Thanks. I love you, Rias."
"I love you too, Issei."
"Ugh..." Sighed Vali. Talk about an idiot couple.
Gaslighting: A form of psychological manipulation in which a person or a group covertly sows seeds of doubt in a targeted individual, making them question their own memory, perception, or judgment, often evoking in them cognitive dissonance and other changes, including low self-esteem. Using denial, misdirection, contradiction, and misinformation, gaslighting involves attempts to destabilize the victim and delegitimize the victim's beliefs. Instances can range from the denial by an abuser that previous abusive incidents occurred, to belittling the victim's emotions and feelings, to the staging of bizarre events by the abuser with the intention of disorienting the victim(from Wikipedia)
AKA: What most authors do when writing 'Manipulative Rias' to Issei and, to lesser extend, you, the reader.