In order of when they get together with Kaito...
Kuromeina (nickname Kuro) - Dark King of the Six Kings, the leading and strongest figures in the demon realm, and also the one that raised the other Six Kings. She's the one that helped Kaito break out of his shell and also the trigger for him getting to know all the world's VIPs.
https://mypage.syosetu.com/mypageblo...ogkey/1680909/
Icesis - Death King of the Six Kings. Very lonely and kind girl, but everyone instinctively fears her because of the aura of death she releases unconsciously. Proposes Kaito on their first meeting. I classify her as a yanderedere because she's merciless to anyone that hurts/bullies Kaito (ex. he nearly got killed by a bear monster and she then proceeded to wipe them extinct in an instant), but obediently listens to Kaito and oozes an aura of love when it comes to him.
P.S. Best girl, best waifu
https://mypage.syosetu.com/mypageblo...ogkey/1806663/
Jieglinde - Elf girl that's the childhood friend of and works for the noble looking after Kaito and the two other highschoolers that were caught up in the summoning. Acts as his bodyguard from the shadows since day 1 and is mute because of past injuries. Despite her appearance, she's actually the homely type and is really good at housework.
https://mypage.syosetu.com/mypageblo...ogkey/1818914/
Lilia Albert- Duke that looks after Kaito and the other two highschoolers. The little sister of the current king of the country they reside in and was leader of a band of knights until she renounced her position of royalty and is also one of the strongest people in the human realm. Because of various reasons, she has almost no immunity to men and is a late bloomer. Frequently assaulted by stomach-aches, fainting, and going hysteric because of the meetings Kaito has and things he does.
https://mypage.syosetu.com/mypageblo...ogkey/1703943/
Alice - Illusion King of the Six Kings. Most feared of the Six Kings due to having agents all around the world in all places, thus having access to all kinds of information and is ruthless to those deemed harmful to the world. Acts like a complete idiot when with Kaito and he's always having to scold/preach to her. Initially only got close to Kaito to test him, but ended up falling for him in the process and from then on prioritizes him over the world.
https://mypage.syosetu.com/mypageblo...ogkey/1838870/
The following have yet to actually get together with Kaito, but are confirmed to do so later on whether it be during main story or in after story in the holiday specials.
Kusunoki Aoi - One of the two highschoolers that was caught up in the summoning. Used to play the a MMO with Kaito since she was in elementary school until Kaito quit 3 years prior for college exams. They never met each other in real life back and figure out it was each other several months after being summoned. Fell in love with Kaito when they were playing the MMO together and only realized after he quit.
https://mypage.syosetu.com/mypageblo...ogkey/1698312/
Yuzuki Hina - The other highschooler caught in the summoning. Cousin and childhood friend of the guy summoned as the hero role and little sister of one of Kaito's classmates in high school.
https://mypage.syosetu.com/mypageblo...ogkey/1739344/
Note: The two above highschoolers get barely any screen time at all because author is a lolicon.
Lunamaria - Maid and close friend of Lilia. Always screwing around and messing with Lilia. Member of the fanatic-cult-like fan club of Kuro.
https://mypage.syosetu.com/mypageblo...ogkey/1710093/
Sharovaanal (nickname Shiro) - Goddess that created this other world and thus absolute leader of the divine realm. Is almost completely expressionless and has no tone to her voice. Gains an interest in Kaito who then becomes the first ever to gain her blessing, thus dooming him to zero privacy ever since she's omnipotent and is watching him 24/7.
https://mypage.syosetu.com/mypageblo...ogkey/1716522/
Eina - The first person to be raised by Kuro and would've ranked along with the Six Kings, but proclaims herself Kuro's maid instead. Superhuman maid that can control time and do practically anything perfectly the first time. Does not know the meaning of restraint.
https://mypage.syosetu.com/mypageblo...ogkey/1722539/
Noin - One of the past heroes that decided to stay in this world and was turned into a demon by Kuro's power. Born in the Taisho era, thus has a very old fashioned way of thought when it comes to romance. Prefers Japanese style meals and provided rice and other traditional Japanese foodstuffs to Kaito and the other highschoolers on their first meeting.
https://mypage.syosetu.com/mypageblo...ogkey/1692494/
Chronoa - Supreme Goddess (the gods ranked directly below the creation goddess) of Time. Goes through the most troubles out of everyone in the divine realm. Is always being treated like a man, so is not used to being treated like a girl.
https://mypage.syosetu.com/mypageblo...ogkey/1824993/
Fate - Supreme Goddess of Fate. Wants to become a neet and is always making approaches at Kaito to get him to look after her. Is so lazy that she starts complaining after walking just 3 steps.
https://mypage.syosetu.com/mypageblo...ogkey/1812032/
The following don't have character designs released yet.
Lilywood Yggdrasil - Realm King of the Six Kings and spirit of the world tree. The most sensible and reasonable out of all the Six Kings and thus the one that goes through the most troubles. Thinks of herself as just a tree, so is oblivious to how Kaito sees her as an attractive woman until said outright.
Anima - The bear monster that Kaito fought and was nearly killed by reincarnated as a demon by Shiro's power and becomes Kaito's servant. Later realizes her feelings for Kaito go past that of a servant and in one of the holiday specials, it's mentioned that she continues acting as his servant even after marrying.
Lazlia - Fairy that's another of many that Kuro raised. Has the mind of an innocent child but is actually fairy king class and can make interact with cause&effect to make her arrows hit no matter where she shoots from. Battle is the last thing on her mind though and is always going around having fun and growing vegetables.
Rindbloom (nickname Rin) - Baby white dragon that got attached to Kaito on his first meeting and considers herself a couple with him. Is working on learning how to transform into human form to be with him.
Chris Diia Fon Alkreshia - Emperor/Empress of one of the countries in the human realm that will even sacrifice her life for the sake of the country. Initially tried to seduce Kaito for the benefit of the country, but ended up falling for him instead. After the main story, in accordance with the rule she put on herself (giving up the throne if she finds something more important to her than the country), she abdicates the throne to be with Kaito.
Noa - Lunamaria's mother and half vampire. Kept refraining from drinking blood since she has a rejection reaction to her husband's blood (who died 50 years ago). After getting a taste of Kaito's blood, it turned out to be a perfect match for her causing her to get addicted to it and falling for him more and more each time she drinks it. Has the appearance of around 14 iirc.
Fiia - The demon lord that was defeated 1000 years ago. Acts as the town doctor and is super clumsy when she's not treating someone. Very open with expressing her love to Kaito after she falls.
Raguna Diia Hydra - King of one of the countries in the human realm and was also a member of the hero party to defeat the demon lord. Wants to abdicate the throne but the other officials constantly stop her from doing so and frequently runs away from work. Appearance of a little girl.
Shia - Serious-senpai's in-universe character. High ranking goddess of disaster. Taste buds are broken to the point that even the spiciest thing in a restaurant known for it's spiciness at 100x spice still tastes sweet to her. Tsundere and has a foul mouth, but actually really kind.
Eden - Chief god/goddess of Earth and is yanyanyandere for Kaito. Fell for Kaito because he rebels against her, so all his denials at her advances only make things worse.
note: This one I don't actually know if she'll actually enter the harem in the end but....considering how this story is...