I guess i should start with explaining the format of the story, so people don't get confused - the original VN has a total of six routes, one per heroine, and a final one that gets unlocked after going through all of the five "normal" routes. Each of the routes has only one ending (There is no "good" or "bad" variation of it, like other titles sometimes employ, the ending is set in stone instead) - by "Erika's ending" we mean the one reached after completing her path. The "True End", as we label it, is the ending for the 6th path, the one which gets unlocked after the previous 5 are completed. It is an Erika's ending as well from the romance standpoint, but the distinction should be made clear - by "Erika's ending" we mean the ending to her route specifically, and by "True End" we mean the ending to the sixth and final route.
Despite having the same pairing, Erika's route and True route don't have much in common though, in terms of plot and focus.
Erika's route, as the name suggests, is largely focused on Erika herself, her relationship with Kouhei and the bloodlust issues. Much like the anime, the actual reasons behind Kouhei's blood being special, and the deeper truth behind the vampires, never gets touched here.
In the True route however, while Erika and Kouhei remain as the trigger for all the events, the main focus is the Vampire Side as a whole, rather than just Erika and her immediate relationship with Kouhei. It is here that many of the illusions get shattered, several truths come to light, the past and present of the vampires and their existence get explored, and Kouhei's "mystery" gets answered.
What the anime did, was basically combine the
premise of both of these routes, but didn't bring either to conclusion. Most of those who had played the game likely expected the "True End" - indeed, that is where the answers lie, and the anime presented several events that serve as prelude for the True route specifically - Kiriha's search for her master during night while giving birth to the vampire rumors; restoration of Haruna's memory - these are the trigger events for the True route in the VN, and never happen otherwise.
Regardless of the ending taken, the inclusion of both is a good choice in a unified presentation - they are good for giving a form of closure (Haruna) and exposure (Kiriha), that would be otherwise absent in face of lack of their specific routes. The fact they included Erika's events from both Her route and True route is no surprise either - she is the main heroine after all, and the content doesn't conflict.
Approximately two episodes ago though, it should have become apparent to the game players that they are not going for the True End - setup and several key events were missing, and there was no real way they could have crammed all that in 2 episodes. So Erika's Ending was the apparent alternative - a surprising choice, given they would not be answering many of the core questions, but i was fine with it - personally i always slightly preferred Erika's ending to the True ending, due to the emotional implications of it. (That, and there was a certain thing in Erika's ending i could easily see related to the "Red Promise" part of the anime's title).
Indeed, the anime more or less followed the development of Erika's Ending up until Kouhei's confrontation with Kaya ... and then went nowhere. I don't think the anime conveyed well the intensity of Kaya's character (never mind her motivations and her not inconsiderable concern for her children, twisted as it is), and that of her objective - to make Erika take a servant. She didn't try to strangle Kouhei, nor make threats about killing him - she went ahead and did it!
Well, not kill him, but it wasn't far - which was the whole point.
There was no time for mushy conversations nor corny convictions - as Kouhei was lying there in a pool of his own blood, Erika had two immediate options - either turn him in to a servant
right now (as the vampiric durability would allow him to survive the wound) or watch him bleed to death within minutes. Obviously not much of a choice, as the second is a complete non-option as far as Erika is concerned. She saved Kouhei's life at the cost of his humanity, making him her servant in the process. A sneaky, unfair and despicable move by Kaya by all means, but can't deny it was effective.
So the next time Kouhei regained consciousness, he was no longer human. Erika was obviously less than happy, but that was the situation they had to work with. Actually, up to this point Erika never had drank Kouhei's blood, and sometime after his transformation is the first time she did so. Though she demanded a promise from him before doing so - should she ever, for whatever reason, issue an order to him, she wants Kouhei to leave her immediately afterwards - abandon her and never appear before her again, as that would be when she would have lost the right to have him by her side. This is what i expected to fit the "Red Promise" part of the title, and thought it would fit nicely. Since this never took place, though, i wonder whether anything in the anime relates to it at all
The story eventually ends with Kouhei and Erika leaving the academy - it won't be easy, and they will have other issues besides their everlasting youth to deal with (like Kouhei's "shutdown periods" that Kiriha experiences), but they have an eternity to make it work.
So ... yeah. This is Erika's Ending, and one which the anime appeared to be going for up to a halfway point in the last episode, so i believe my disappointment is understandable
It is no less of a shame they cut out their romance - originally Kouhei and Erika were in a relationship long before the closing events took place (which also likely contributed to Kaya's realization Erika would definitely take action should Kouhei get mortally wounded), and it is kinda frustrating that was removed as well. Can't even really call the anime "romance", because there was none. Aside from all the missed quirks and fluff (like Kouhei's room eventually containing so much Erika's stuff their tea party companions were wondering if they shouldn't relocate
), they also missed the opportunity to give several hints/implications regarding to the bloodlust, as Erika's condition actually improved after she started going out with Kouhei.
Overall they made Erika more 'secretive' regarding her bloodlust, and Kouhei more oblivious to it (for example the pool scene - originally she doesn't hide the fact, and reassures Kouhei it isn't that bad this time - even though Kouhei offers to let her "have a bite") ... i honestly didn't like the recurring theme in the anime with Erika having constant fits because of it, but blowing it off as nothing, and Kouhei appearing ignorant of what should be the obvious cause. I certainly didn't feel the bond and trust between these two, with Erika never talking about her issue, and Kouhei seemingly in blind denial with his "Vice President (how about you actually start calling her by her name !?) doesn't need that!" ...well, yeah, sorry, but she does, and you were originally supposed to realize it long ago
So ...yeah. "Play the game for a satisfying conclusion" is all i can say, and if that's what the anime was trying to convey, one might argue they succeeded.