@Blue-eyes shining Drake
Your take on Ais is way off base dude. Please, stop self-inserting for a minute and try to take the time to understand Bell and Ais. You have totally forgotten and/or haven't even bothered to find out about what Ais has done for Bell. You are also making massively incorrect assumptions on Ais's character without bothering to even try to understand her. Ais was bound by her promised favor to Freya in MS18. Freya allowed Ottar to train Ais for a week, which was a huge favor. Omori also described it as binding, with Ais even feeling like a chain had been placed around her neck. Ais thought there wasn't anything that Freya could possibly want from her since in her mind, Freya had it all. Little did Ais know that she had the very thing Freya wanted, Bell's heart.
Right after MS17, before Ais could interact with Bell, Freya directed Ais to not involve herself with ANYTHING involving Bell until the end of the war game out of jealousy because she couldn't bear to see Bell and Ais get closer in her mental state at the time. Freya compared the thought of them getting closer like a rusty dagger stabbing her heart.
Freya had Ottar deliver the conditions of the favor to Ais at Freya familia's home. Ais acted like she was going to back out as Ottar indicated it was a just a verbal agreement. When Ais asked if it was okay, Ottar told her that a sword that couldn't keep its oath would surely one day become dull and shatter. This statement completely stopped Ais and she looked up to the sky and she could only squeeze out one word, "Bell", while thinking she was sorry. Now, Omori hasn't explained or followed up on exactly what Ais was thinking at the time, but we can make a very good, educated guess. Ais has been compared to a sword numerous times throughout SO, with herself even making the comparison. So, we can assume that Ais was thinking of herself when Ottar said that a sword would go dull and shatter that couldn't keep its oath. Ais can't have that happen because her number one priority is to take back her mom and get revenge on the OEBD. She has to be strong to do this. Ais has made an oath to herself to take back her mom, so she likely thought that her disregarding Freya's favor would affect her ability to keep her oath to save her mom. This is further backed up by the fact that she looked up to the sky after Ottar finished talking. When she made the promise to take back her mother and when she was thinking about her, she also looked up to the sky. This is probably because the OEBD flew away with her mom.
We can't expect Ais to put Bell as first priority yet, because that would be out of character for her since she has been obsessed with her wish since that fateful day when the OEBD took everything away from her and caused deep trauma. Ais isn't perfect and it isn't realistic for her to be perfect. She has flaws like normal people. Ais hasn't had her arc yet and she still has a lot of development to occur in MS. Did Ais's decision to not help Bell hurt her greatly? Yes! As RabidPotato pointed out, when Bell was being beat to a pulp by Ottar, she dug her fingers into her hands so hard that she bled while cursing herself over and over again for not being on the battlefield. It was incredibly difficult for her to watch the wargame and Tiona had to help her get through it. Later on when things were going better in the wargame, she was shown cheering for Bell and even shouting his name while watching in the mirror. Ais probably showed the most emotion in MS18 that we have seen her show, all surrounding Bell.
Bell wasn't bothered in the slightest that Ais wasn't able to train him because he understood that there were circumstances as to why she couldn't. Tiona told him that she was sure Ais would be cheering him on which touched his heart. He was happy just knowing that Ais was rooting for him (which she was!).
So where does Bell's well-being stand on Ais's priorities at this time? Probably right at number 2. This was demonstrated in MS16-17 where Ais helped Bell by fighting Freya familia, disregarding any possible consequences of familia relations. She even said that "none of that matters" when Van threatened to bully Bell. Ais demonstrated multiple times in MS16-17 that cares deeply for Bell and is willing to support him. When Hestia broke Freya's charm, who was the very first person to arrive at Freya's manor to save Bell? Ais. In fact, she saved Ryuu just in the knick of time in the process.
Ais also played an indispensable role for Bell in MS17. I encourage you to read MS17 to fully understand it, but basically Ais saved Bell from mental destruction and from losing his identity as Hestia familia's Bell Crenell when he was at his very lowest point in the whole series. Ais was the only person that Bell interacted with while charmed that didn't reject him and the only person to remember their interactions with him. This meant the world for Bell and his knees gave out in release, in hope and he cried shamelessly like a child while holding Ais's hand to his forehead. He renewed his feelings for Ais and told her that she is the one he admires and it wasn't wrong at all to meet her (reference to the very beginning of the books when he was saved by Ais). Bell then set out to confirm the miracle Ais had given him, to confirm the path he walks. He did so by fighting better than ever against Hedin, using Ais's training to finally be able to land a scratch on Hedin. After the training, while reminiscing, he noted his body was filled with feelings for Ais and he was consumed by a sense of exaltation. So is it that hard to imagine why Bell wasn't bothered in the slightest that Ais wasn't able to help him in the war game in MS18? He most likely believes she had a good reason.
I won't go into detail all of the other stuff that Ais has done Bell. It is pretty ridiculous that you mentioned her only one moment of selfishness (pre-MS18) when she wanted to train Bell in the first place. She felt extremely guilty about it and did her best to make up for it by giving it her all when training Bell. Bell and Ais treasured the time they spent training together. Bell said it was like a dream come true. Ais embraced the irreplaceable time with Bell deep within her heart. That training has been the foundation for Bell's adventuring career and has saved him multiple times throughout the series.
Now for the rapid fire list of Ais helping Bell, look them up if you want:
Ais saved Bell from the minotaur. (MS1/SO1)
Ais saved Bell when he went mind-down in the dungeon after overusing firebolt. (MS2/SO2).
Ais saved Bell when he was fighting the ogres. (MS2/SO3)
Ais returned his vambrace, apologized for the hurt caused by the stray minotaur, and offered to train him. (MS3/SO4)
Ais trained Bell on the wall for 6 mornings and one day. (MS3/SO4)
Ais got to Bell just in time when he fought Asterius, telling him he fought well and she would save him, giving him the fire/motivation to take his first step towards being an adventurer and being a hero. (MS3/SO4)
Ais saved Bell on the 18th floor and took care of him for the whole two days while he was staying at Loki camp. (MS5/S05).
Ais and Tiona train Bell for an entire week around the clock in preparation for the war game. Telling him it would be wrong to abandon him. Bell understood it was a huge favor and it could have serious consequences for her. This training is what, of course, enabled him to win the war game. (MS6)
When Hestia is kidnapped, after seeing the panic looked on Bell's face, Ais offers to find and saves Hestia despite Loki telling her that it would be a huge pain in the a**. Ais spends 5 days in Edas village watching over Bell and Hestia. (MS8/SO9)
While at Edas Village, Ais finds out that Bell and Hestia had a fight. She attempts to counsel him and fails miserably due to her awkwardness, but it showed she wanted support and be reliable to him. (SO9)
Ais pulls Bell aside upon seeing at Babel when she notices he is distraught. Her advice didn't help him of course, since she was on the polar opposite of what he was thinking but it still demonstrated she wanted to help Bell. (MS10/SO10)
Despite Bell being on polar opposite of her belief system and it nearly breaking her mental state, Ais admits that Bell and Wiene were right. Upon recovering from her depression, she goes to see Bell and agrees to train him again, giving Bell the motivation to keep running. (MS11/SO10-11)
Ais agrees to help Hestia take back Bell from Syr, she ends up in several fights with Freya familia in support of Bell. (MS16)
The next morning after Bell rejects Syr. Ais remembers the sad state Bell was in and couldn't leave him be and went out looking for him. When Freya familia attacked Hestia familia she rushed to support him, being stopped by Freya familia before Freya charms the city. (MS17)
Ais remembers her interactions with Bell and did not reject him when he came to talk to her while everyone in the city was charmed, saving him from mental destruction. (MS17)
When the charm is broken, Ais is the first to arrive at Folkvangr to save Bell. She saves Ryuu in the process. (MS17)
Anyway, that is a quick and dirty list. Those interactions are covered in detail in SO and MS. They all have heavy emotional weight for Bell and Ais. I suggest you read and understand them before commenting on Ais or at the very least stop spreading nonsense. I think taking time to understand her and why Bell x Ais is a thing would help you enjoy the series more. If you earnestly do, you won't be surprised at all why Bell is so set on pursuing Ais. Heck, you might even change your mind about Ais. Also, please note that this list wasn't an exhaustive list of their interactions. It was only the ones highlighting Ais's support for Bell. There are a bunch of other interactions that build up their relationship.