In Ryuu's case, it won't matter if Bell is stronger than Ottar or not. As Ottar already showed during his training with Ais and his battle against Freya's other executives that level 6s don't pose any threat to him, I assume that Bell will be a pseudo level 7 in volume 18 in order to defeat Ottar, just or accompanied, and if this is the case, when Ryuu joins the family as level 5, she will already be weak compared to Bell.

So Ryuu could be of great use and support to the Hestia family in general, but in the particular case of Bell, it will be another companion that he must protect, that's what I meant by saying that, although I'm glad that Ryuu joins, she really won't be as valuable as you might expect.

That's also why I said that for the Hestia family to be a real support for Bell, it will take many more strong adventurers like Ryuu, to make up for the level gap.

I am still excited about Ryuu joining because Bell needs her badly due to his level.

The reason why I wanted Ryuu to join the Hestia family is because I thought that she would be Bell's partner in his process of becoming as strong as a level 6, that is, I thought that she would rise to level 5 and together they would live different adventures that would make Bell as strong as level 6, in this process Ryuu would be very important and essential.

However, Bell went from being a "level 4" to being a "level 6" almost instantly (in volume 17), so this process I mentioned didn't happen and now that Bell will be a "level 6~7" I don't see very clear how Ryuu could be of great use to Bell.

There is an essential service that she will be able to provide as a caretaker, if Bell goes on a dangerous mission with the Loki or Freya families for example, where he obviously won't be able to take his weak family, he will be able to be calm knowing that Ryuu is taking care of them.

Emiya, you think Bell will need Ryuu very much, so how else do you think Ryuu will be able to be very necessary to Bell considering the 2 level gap between them?

Don't think that I am rejecting your point of view, I just want you to develop your idea further.

Bell is not a pseudo level 7 he stands currently possibly as a strong level 5 bordering to six.

This is still not enough to beat Ottar even with Argonaut fully charged .

Bell has no chance alone in beating him is realistic to beat the other level six from Freya but not the Warlord himself.

To beat Ottar requires teamwork in the same manner as fighting a very powerful monster.

How strong Bell is is somewhat controversial, there is a Omori's tweet where he said that Bell with his status as it is at the moment he already has the power of a level 6, he also said that Bell lacks a status update as a level 4 where his increase is expected to be massive considering that it was in chapter 5 of volume 17 when the "training" intensified, adding these factors, Bell as a level 4 could well have a power of level 6 medium low.

Then there is the fact that he can go up to level 5 from volume 15 and also it is expected that as level 5 he will have a super training with the Loki family where his status will increase enough to be a pseudo level 6 almost 7, in order to of being able to defeat Hedin by himself.

All these theories have been discussed a lot here on the wiki and although I don't like them, they are the most accepted.

So if things are such that Bell can defeat a level 6 on his own, Ryuu even as a level 5 will be weak to him. It is logical to think that Bell will need a team to defeat Ottar but I don't think Ryuu will be part of it, I assume that his teammates will be Finn, Gareth and Riveria and maybe Ais too, but remember that Ryuu was defeated in one hit by Ottar and her power can't increase massively in a single volume, she's not Bell, so I think Ryuu won't contribute against Ottar.

Oomori has a messed up system Bell is listed on paper as a level 4 currently.

You can’t suddenly jump two levels normally is a level at a time when a adventurer promotes is one level ahead not two.

it was never said or shown that Bell was close to level 6.

I still think that is a load of crap the level of Bell even with his stats has no chance of beating Ottar by himself.

If the author believes this then his system of Falna in DanMachi should be thrown away.

I read the brief sypnosis of v18 listed on this site Ryuu is leaving to meet Astraea.

There is no doubt now on the reason for this only a deity can release a member to another Familia.