Could Paul of Taurus be an example from the Bible, since he's an Apostle that came after the original 12?

Do they have to join the team to qualify as Sixth Ranger, or just work with them? For instance, Casey Jones from TMNT seems to fit almost perfectly, but he was more of an 'outside consultant' than an actual member of the team. ~~~~

Would Captain America fit as an example of a Sixth Ranger? After all, he didn't join the Avengers officially until that legendary No.4 issue, and he was a near-permanent Avenger since that point.

Was there a trope where Sixth Ranger could be applied to a group? (i.e. A new team joins an established force after becoming independent/leaving another force.)