Revision - I had already finished writing the documents a while back, but I now thought that giving a general overview would be good. Once you click on either the word Pastebin or Google Docs you’ll be redirected to the main page, which contains very relevant instructions, so please read it first. This will lead to a complete understanding of the heart of my answer to your question: Quotes from Drama CDs, Quotes from Side Stories, Quotes from light novels (volumes 1-4), Quotes from light novels (volume 11). After that, there are the final conclusions on Shalltear’s love for Ainz.
As a side note, I recently answered another question in this forum about Overlord’s translation, so I decided to also edit these files so that they include the comparison between the official translation and the fan translation in more detail.
If you just wait for a few days, I'll give you many pages worth of such sentences as well as a full analysis on Shalltear's love for Ainz. I'm linking my own Pastebin and Google Docs for that purpose. (Right now they're visibly empty, but it will take me very few days to gather enough material to create a very long document)
In the meantime, I will explain one of quotes from Bhavin as it's quite revealing. I'll also quote a conversation between Shalltear and a death knight from the side story "Emissary of the king" that the author wrote.
This sentence that Bhavin mentioned (from volume 2 of the light novels) is very beautiful.
この = These
二人 = two people
の = possessive marker
対立 = opposition
は = subject marker
恋敵 = rivals in love
であること = the fact that
に = from
起因している = originates
These two's opposition originates from the fact that they are rivals in love.
アインズ = Ainz
は = subject marker
アルべド = albedo
と = and
シャルティア = shalltear
両者 = both
から = from/by
惚れられていた = to be in love with
About Ainz, to be loved by both Albedo and Shalltear
絶世の美女 = peerless beauty
二人 = 2 people
に = from
愛される = loved
これ = this
を = grammar marker
喜ばない = not be happy
男 = man
は = subject marker
いない = doesn’t exist
だろう = most likely
Loved by two peerless beauties, to not be happy of this, most likely there doesn't exist a man.
The reason the sentence is so beautiful is because the author used a different kind of love each time, showing how complete Albedo and Shalltear's love for Ainz is.
恋敵 (koigataki) = it uses koi, which can only mean romantic love. They fell in love with Ainz in a romantic way, the way you would feel for your boyfriend /girlfriend. It is always 100% romantic.
惚れる (horeru) = to be in love with/to be infatuated with or consumed by (romantic.) The romantic feeling is very strong on all occasions. The usual translation would be to "fall in love with" but context should be taken into consideration. The next love word used makes it clear it means “to fall in love with” with a very strong feeling of infatuation as well.
愛される (aisareru) which means a very deep type of love (to be loved by.) The word "愛" is the standard word for love in Japanese. However, Japanese VERY rarely say this word, and some people go their whole life without saying it. So it has come to have quite a deep meaning, you will almost always see suki used in the place of ai. Unlike the other 2, this type of love doesn't necessarily have to be just romantic when referring to another person, though it's even rarer to use it for family or a friend.
So far, Albedo and Shalltear are the only floor masters that have shown the first 2 kinds of love. The third type of love should also include romantic love in their case.
Regarding the conversation I said I was going to quote, it is from the side story "Emissary of the King" that Maruyama wrote. Ainz sends one of his servants, a death knight, to ask the floor masters what it is they really desire. Shalltear is a bit skeptical at first.
"Then… Is the reason you came to me first because I am the furthest away from Ainz-sama? Was it purely a matter of distance?"
It wasn't like that. The Death Knight told her what his master had said, that "Shalltear is a very important person." The mood changed in an instant. Something that felt like hot air roiled in front of him. There was no change in Shalltear — no. Her crimson pupils glowed like lava.
"—Forgive me. I must have gone deaf for a moment, so I didn't hear what you said just now. Do forgive me. Could you please tell me what Ainz-sama said, one more time?"
It was hard to believe that undead, with their immunity to biological status effects, could go deaf. But he had no reason to deny Shalltear's request. So, he repeated himself.
The corner of Shalltear's mouth turned up. "—Ha! Hahaha! Ahhh, if only Albedo was here to listen to this! Hahaha!" After having a good
laugh, Shalltear continued asking the Death Knight other questions.
"Did Ainz-sama say anything about Albedo?" After hearing that Ainz hadn't mentioned anything, Shalltear was all smiles. ... Conversations were like passing in basketball. If the exchange ended after only a couple of rounds, something was wrong. … Seeing Shalltear's discomfort, the Death Knight asked the final question, "What do you want?"
Shalltear's smile was bewitchingly beautiful as she made her reply. "I want Ainz-sama's love. I don’t mean to say it only has to be me. After all, it's normal for a great man to claim many women for himself. It would be surprising if any woman could resist an absolutely amazing man like Ainz-sama. However, I want to be the first in his heart."
(Later, Ainz-sama’s servant goes back to Ainz and tells him what happened and what she desires.)
"Very well. Then let me hear the answer to the last question." As he heard Shalltear's answer, Ainz sagged like a puppet whose strings had been cut. "... Besides Ainz. Append that to the last question the next time you ask it. Understood?"
(Later, the death knight servant goes to Cocytus and asks)
"What do you want… besides Ainz-sama." Cocytus seemed to grin at the addition. "…I. See. Someone. Asked. For. That. It. Must. Have been. Albedo. Or. Shalltear."
(Each servant wants a different thing: Aura wants magical beasts, Mare wants plants… On the other hand, Albedo and Shalltear love Ainz too much, and they may never get their wish granted.)