The major indicator is that we are very likely in the final third of the story, and the mystery of Ais' backstory is the biggest plot thread remaining, besides the more obviously endgame stuff like what is at the bottom of the Dungeon, or actually defeating the Dragon. Because of that I imagine we're only just a few volumes away from Ais' arc starting in earnest.

Ais' personal character conflict has been that she's always wanted someone to save her, but is now so strong she feels that can never happen, so for the arc to culminate in Bell finally reaching her and becoming close to her equal, feels the most thematically relevant.

And a big theory I and I've seen others share too is that the OEBD possesses some kind of connection to Aria, possibly in a similar way to the assimilation the spider creature used on Artemis in the movie, and that Bell and Ais' main roles in the battle will be to sever it. Thus, could reasonably see them doing that as Level 6's, since I also don't expect the Dragon fight to happen that long after Ais' arc is concluded.

Unless Omori double levels Bell he will be around mid 6 to 7 by the OEBD arc.

If you take Omori at his word there are 2 major arcs post LN20. Bell hasn't been generating much excelia over LN19 so he will start the Ais Arc as a mid lv5 with Asterius probably being his feat for lv6.

OEBD is next and he will probably mid lv6 when they start the serious part of OEBD fight.

The problem is that Ais specializes in killing monsters, so even with his status excess Bell is not even close to match the combat abilities (it would be a different story if she was fighting people but after the faction war, there is no familia left who can pose as threat) Ais have against monsters. This is a big part of why Ais was sidelined as she could have soloed 90% of the threats Bell faced. There is a reason the Ryuu Arc started right after Bell & Ryuu were on the same level.

Addtionnally, we're still waiting for this promissed training session since volume 11 who was reconfirmed in volume 17 which is obviously a special progression card from Omori to take Bell from mid to high in no time. So technically it only require 1 book for Bell to level up which can happen in Volume 22 if Omori decide to adapte the Grand Day Event.

Agree that Ais arc would start when Bell becomes a lv6. Though I genuinely wonder if that training between Bell and Ais is still gonna happen when Bell now has Hedin and other Freya elites to train him.

To me it feels like there's two outcomes: for the scenario when he becomes a Level 6 after the Ais arc, he doesn't confess to her because he knows her past and her feelings about the OEBD, so puts it to the side so he can help her deal with the Dragon first. Or, in the scenario where he becomes a Level 6 before the Ais arc, the impetus becomes him confessing or trying to confess to her, which spirals into him learning her history.

@Longhatruong I don't think there is any doubt that it will occur, it just a question of when Omori wants it to happen since it would be absurd for Ais to not keep her promise with Bell after she kept her promise to Freya. Especially since Ais showed in 17 that she really wants to train with him and Bell would love to just spend time with her. Not to mention Bell will always get the most growth from training with Ais. None of the Freya executives or Ryu can substitute Ais's role. Anything she can teach him is a bonus at that point. I suspect the training will be used more for developing their relationship since that and delaying Bell's growth are really the only two reasons I can think of as to why Omori is delaying it so hard.

Also, I expect Bell to be 6 for the final arc and maybe hit 7 before the final battle. I just can't see Ais's arc being anything other than OEBD arc since her history is directly tied to it which will probably be the last arc.

@Jtg1111111

About the promise, it's more like Bell will be the one who need to ask Ais out to train him again and Ais would of course agree, instead of Ais will be the one having to ask him out herself. That's why I wonder if Bell would ever go do this when now in the Hearth Manor, there's a bunch of masters to train him. Yes, Bell gains the most from Ais due to his skill, but Bell doesn't know this. To Bell, Hedin and the others can all properly teach him. Of course, I know if Omori want it, it can happen, but doesn't make me what sort of scenario can it happen. Just like how everyone wonder if OEBD could be beaten even though it's gonna be defeated for sure.

About that deal between Ais and Freya, personally I have my own thought about that. It's not a problem of honor. I don't think Ais had any sense of obligation that she needed to keep the deal with Freya no matter what or that she felt like she heavily owed Freya. She fully intended on attacking Freya Familia in vol 17 and also wished to back out of that deal if she can. It's just that as Ottar said, her sword will rot away if she can not keep her words, and she can not allow that. Because to her, she is a sword and she has an oath to fulfill, one more important to her than anything else. From how she view Ottar's words, she's incapable of keeping her words would also lead to her incapable of fulfilling her dearest wish. That's why she backed out from Bell's affair. Ais ain't someone so honorable who says something and feels the obligation to absolutely follow it. She simply did what's she thought was the best for her.

Bell does have access to capable trainers, but that doesn't really matter since Bell would still want to train with her and specifically her even if it was just to spend time with her since he loves her. It'd be the easiest way for them to do so. Besides, from Bell's point of view he would probably still think he can learn from her since she is the one he admires which is actually probably true. In short, I just don't think Ryu and Hedin being able to train him really changes anything.

Really it is in Ais's court since she had circumstances as to why she couldn't train in both V11 and V18. Ais has shown it multiple times that she really wants to train him, so it'll happen when Omori allows it to happen. One of them just needs to broach the subject when they finally interact one-on-one again which hopefully happens in V20 since Omori had Tiona say Ais would come to find him.

As for the favor, it probably came down to her thinking if keeping her word or not would affect her ability to fulfil her goal to take revenge on the OEBD and take her mother back (which is why the sword statement from Ottar stopped her in her tracks). She didn't do anything wrong by fulfilling her obligation to Freya since it was a really big favor to let her train with Ottar that ultimately saved her life in SO12. Obviously wasn't easy on her though as shown by her reaction to Bell getting the crap beat of him by Ottar and I think it will help her realize some things in the long run. Bell is someone who can't go back on his promises either, so ironically Bell would have done the same in her shoes and he will understand if she ever talks to him about what happened.

To me Bell stands a chance to defeat the Black if he reaches at least level 7.

The most rational conclusion is that the series will allow him only to level six the deadline is fast approaching.

Bell needs to be level 7 by that point, no haruhime level boost will improve his odds until then.