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"A useful member of the team whose departure makes things worse" strikes me as being too general.
Most people on a team or working a job have a tendency to be at least competent and useful, or else they would be removed from that team. In fiction, you can sometimes have a character be wildly incompetent (The Millstone, The Load) and nobody will Just Eat Gilligan because the story says so.
(I'm not saying you can't have unpunished incompetence in real life, but there's usually a reason like Nepotism, The Peter Principle, or just plain old apathy.)
If you meant someone who hypercompetent and is indispensable, there are several directions to go. The Ace, Master of All, Story-Breaker Power, or their departure being a Deus Exit Machina. Probably a couple more out there.
Edited by KD
Vetinari Job Security might be pretty close.
Agreed with KD that "a member of the team is competent, and the rest of the team is less functional in their absence" is, by itself, just People Sit on Chairs. But specific types of competence can still be troped. Aside from what's already mentioned, there's:
- Hyper-Competent Sidekick or Almighty Janitor: where a character that you'd expect to be irrelevant is actually the most competent one.
- Bunny-Ears Lawyer: a team member who initially seems useless is actually hyper-competent at their one specific job, justifying why the team keeps them around.
- The Heart: the team member who mediates between the rest of the cast, sometimes single-handedly holding the team together. An extreme version of "their departure makes things worse" where the team completely falls apart in this character's absence.
Mmm, wonder if there's room for a complete inverse where there's only one character on the team who pulls their weight, and the others are all millstones / loads.
Edited by WarJay77 Currently Working On: Incorruptible Pure Pureness
Sinc the description of The Millstone is: "A useless member of The Team that actually makes things worse than if they weren't there." I'm looking for the exact opposite: "A useful member of The Team whose departure makes things worse." Since it's been appearing in several manga adapted from Light Novels (like Banished From The Heros Party and Beast Tamer And The Cat Girl), I think we have this trope somehow.
Edited by VelvetPierrot