I will reply (very quikly) to this.
If we analyse the pattern of the strawhats we can say that to be validated for a character there is multiple parameters that must be checked first:
1. As been anywhere in the story a need/want for the post
OR said crewmate to this post or as a member of the crew. It can be very subtle with hints or more obvious with strong setup. The need can also be narrative (for example the need for a first mate is narrative, not explicite)
2. Does the post fits a place on the ship, something significant and does this place make the crew better in their journey ?
3. Does the post has been introduced yet, either in a subtle manner or obviously ?
4. Did said character has been introduced already with capacity for the post
5. Did said character had a strong action related to this post (what I call
"the Shining mugiwara action") and does this action had been witnessed by at least one member of the crew.
Let me give you an example: Jinbe / Helmsman
1. There has been hint all over the story that someone would need to fullfill the job. It can been seen - for example - just before arriving to the whole cake island where Brook take over the helm from Luffy who is doing a poor job at it. You can also say that Jinbe is a crewmate needed by Luffy from Fishmen Island.
2. The post fits one of the most crucial post of the ship, in battle it can make the difference between life or death.
3. The post has been introduced all over the story through the strawhats taking the helm
4. Jinbe has been introduced has an helmsman after Impel down.
5. Jinbe's mugiwara action is the "Green Room", he uses his capacities to get the ship away from Big Momo through the wave. The action amazes all the strawhats present.
Why are those parameters so important you will ask.. Well because everything in a story must be setup first. In another words gradually introduced to the reader. It must stay clear without being obvious. That's a fine line that each author must follow when they are introducing a team. Oda is no different. each of those steps are important.
- The first one introduce the need for the role to the story, for that reason there are job on a ship that will never be shown.
- The second show the importance of said post on a ship, making it relevant to the story
- The third shows that the strawhat are perfoming this role already, therefore this role has a meaning in the construction of our crew
- The fourth introduced us to the capacity of a character. It permits that the author will be able to make him do a great action without having to explain the history of the post for the character to the readers (show don't tell)
- The fifth ciment the character to the role on the ship by giving him a "shonen action", an action so magistral that it amaze one or more character present during the action.
Check it out. All the strawhats have those 5 steps. Not always at the same magnitude, sometimes the shonen action is not that impressive (this is the case for Robin for example) and sometime the need is very subtle, but those steps are always there and serves each characters into there role.
Now.. when we look at it only ONE job has been given the 5 step treatment and it's the Sentinelle/Recon/Look-Out. I won't give you more information, I'm sure that you will be able to find.. in the story, the 5 steps for this specific role.. and unto which specific character it will lead you to..
