Basically, I'm of the mind that the "real/true" Raynare - or perhaps better phrased, "what she's like under normal circumstances" - is best exemplified by her behavior in the scene where she kills Issei, not by her scenes after the fact. Is that scene where she's at her least malevolent/bitchy outside of the false "Yuuma Amano" identity? Yes. Is treating that as her normal personality suspiciously convenient for shipping/waifuing purposes? I confess that indeed it is, BUT! There's a method behind this madness, as I shall now outline.
In the scene where Raynare murders Issei, for the most part she seems quite cheery and even cordial. She tells Issei that for whatever it's worth to him, she did enjoy their date together, and IIRC she's even still wearing that little wristband he bought her. At this point, when he's already halfway bled out and Raynare has no real expectation of seeing him again, it seems incredibly pointless that she'd lie to him, especially if the lie she were telling seemed meant to end their interaction on a positive note. If she were naturally as bitchy as she is later, it seems more likely that she'd have taken a moment to kick him while he was down right then, possibly even literally. Instead, as I said a moment ago, she still seems pleased that he bought her a gift, and she tells him that she enjoyed the date.
Do I take that to mean she actually liked Issei? ....well, if she did have feelings for Issei at that point, evidently they weren't strong enough to stop her from killing him. Personally, I like to think that if Raynare hadn't thought she was supposed to kill him, then she would probably have been content to continue the charade, and it's possible that some stronger, genuine feelings could have been born from that later on. A case of "becoming the mask", if you will.
Needless to say, she did kill Issei when she did, but I again point out that at that time she didn't seem to have any vitriol or malevolence about it. True, she did have a slasher-villain smile immediately before her transformation, but that just confirms she's probably a sociopath of some kind, which is separate from my point. Maybe she even enjoys killing, I'unno, she certainly didn't show any signs of regret for killing Issei. But, again, she left him on what (I think for her/coming from her) is a positive note. She enjoyed the date, kept the present, and would have had little or no plausible reason to lie to him at that point. Not that she's above lying, far from it, but that lying to Issei at that point wouldn't have gotten her anything, wouldn't have achieved anything. Why go to the effort of hiding the truth when there's no benefit to it? (again, I'm aware that the JP text is different, and I don't care. It doesn't even impact the plot, unlike the Rias/President thing)
The next time we see Raynare is after Issei's first meeting with Asia (which was also quite like a date in many respects, even if neither of the two involved were thinking of it as one), and by that time Issei's been resurrected as a Devil, an "unclean scourge/enemy of God", a "creature inherently of sin". Even though they've, well, fallen from grace (hence their fucking name), the Fallen Angels do - at least, according to Rias, but I'm not aware of too much in canon that would contradict her - still, in their own flawed, twisted way, desire to serve God. They use Exorcists and Holy Light just like the Angels do, and more than anything else they appear to hate Devils. This is the important thing, because for all we know Raynare's just seeing Issei again after she left him for dead and even went to the trouble of saying something nice to him, but a) here he is still alive (how inconsiderate!), b) here he is on what looks suspiciously like a date with another woman (and Raynare does call him out on that, in a rather mocking/sarcastic-sounding way), and c) "not ONLY did he not die when I killed him, and not ONLY do I find him with another woman, but that two-timing bastard went and aligned himself with those filthy, digusting DEVILS?!"
I mean, if I were a religious zealot, I could kind of see where she'd be a bit salty about that last point on top of the other two. Admittedly I don't recall much of this scene as well as I do the prior one and what comes after, but the main point is that Raynare is visibly bitchier (or more aggressively bitchy if you're of the mind that her "I enjoyed the date" thing before was sarcastic). Still, at this point Issei isn't visibly hanging around other Devils, and he clearly doesn't have much of a clue what Asia's deal is, so maybe he'll end up a Stray and self-destruct or something. Either way he's an irritant but not a priority (especially when compared with securing Asia in order to steal her Twilight Healing). And you know what? In some still-rational corner of Raynare's mind, it might not even be Issei's fault he's a Devil. Could be that some random Devil came along at the wrong (or right, depending on perspective), saw a dying human with a Sacred Gear, and took advantage of the opportunity to make Issei their servant. In which case killing him again and for real might actually be a mercy, but one that can be held off on until later (because again, Twilight Healing is more important at the moment).
The next time we see her after that, IIRC, is when Issei and the ORC mount a rescue operation to invade the Fallen's church and save Asia, and it's at this point that Raynare completely and totally loses her shit. Compounding everything I mentioned above, Issei is now actively working with Devils and showing the other Devils camaraderie and loyalty as they all invade a House of God to subvert a scheme hatched in the name of His glory to aid the war effort against His Enemy.
The other Devils on the scene, that blonde pretty-boy with the sword and the little girl with kung fu? Eh. Devil scum acting against the Servants of God (Fallen from His Grace though they may be) just means it's Tuesday.
But Issei? A guy she dated (even if it was only once, while under disguise, and she killed him afterward)? First he comes back as a Devil, first Raynare sees of him is what looks like a date with some stupid virgin of a blonde, then he turns out to be a loyal and willing Devil leading their own twisted little parody of a Crusade against HER hard-plotted scheme?! At this point, Issei has transcended beyond being just 'the enemy' for Raynare, he's gone beyond the pale and become The Adversary in her mind, Raynare's very own personal Satan. She's embarrassed, she's humiliated, and above all else she's pissed, and Issei just turned himself into the ultimate "Acceptable Target" from her perspective. This is where we see her at her most sadistic, her most cruel, as she uses everything she can to tear Issei down after "what he's done", after "what he's turned into". He's a Devil, he's God's enemy, he deserves every ounce of emotional torment I can possibly inflict on him by nature of what he is. And this is where she actually does get a benefit out of deliberately crafting her every word and action in a way to cause pain, up to and even including lying (even of the "declaring previous truths to also be a lie" variety). The fact that she lets the rush of finally having her own Sacred Gear go completely to her head and she basically goes drunk with power doesn't help the situation on any level.
It's at the end, when she's desperately begging for her life and even using the Yuuma Amano form and voice to try and stop Issei from killing her, that we see another true of what Raynare is - bereft of any options, too weak to actually change the outcome herself, she's a coward, utterly shameless in her willingness to play with someone else's feelings (but again: "Issei's a Devil, he deserves to be hurt. I'm still a servant of God, so I should live."). But the thing that gets me is that even at the very end, she still had the present he bought for her (or remembered it clearly/vividly enough to produce a perfect illusion of it on the spot and under high stress, which would arguably make it sound like it actually WAS really important to her, and could put her previous "Woman Scorned" bitch fit in a different light).
I'm not at all saying that Raynare's a great person, or even a good one. She's quite plainly deceitful, manipulative, covetous of power and probably status, ambitious enough to doggedly pursue those goals, amoral (or holds herself to enough double standards from being a self-proclaimed agent of God's will as makes no practical difference), arrogant, cowardly in the face of defeat, petty, vengeful (not just with Issei; after getting TH she also gloats about how she's going to get back at everyone who ever "screwed [her] over", and sounds damn near orgasmic at the prospect), and can be staggeringly cruel.
But hey -- even horrible, evil people can fall in love.
