Biography
Bob is a bit off. He might be a helpful member of The Team, but his habit of breaking into song and dance, or his obsession with rulers, or insisting that he carry a carrot of no less than 6.12 millimeters (and no longer) in length at all times, or saluting whenever the air temperature drops below... you get the idea. Put simply, he's hard to understand, talk to, or otherwise work with for everyone except his (long-suffering, but inexplicably loyal) friend Alice.
This trope is about someone who has the task (be it as a friend or their actual job) of keeping an eye on the resident Cloudcuckoolander, Fish out of Water or other strange person. They might be a close friend, relative, hired bodyguard or nanny or even just a plain old servant. As long as they accompany someone who's a bit crazy and try to prevent things from getting out of hand, they're this trope.
Services this minder can provide include:
Generally keeping them out of trouble (calming down angry locals, posting bail, finding directions to the nearest stationery store or farmer's market, etc).
Doing the legwork they're too crazy to do themselves.
Acting as a "tour guide" to explain things to them (this can also be a good reason to make them Mr. Exposition).
Translating what they're saying from whatever their preferred form of communication may be.
In Loyalty Tropes, they might be The Champion.
Of course, since there needs to be a plot (or at least some gags), Failure Is the Only Option. If the person in question is dependent, then this is The Caretaker. Expect their roles to be reversed at least once.
Related tropes include Straight Man, Beleaguered Assistant, Nerd Nanny, Battle Butler, Savvy Guy, Energetic Girl, Old Retainer, Only Sane Man, Only Sane Employee, The Caretaker (who looks after someone who wouldn't look after themselves), Living Emotional Crutch, Translator Buddy (a friend who translates for him), The Reliable One, and The Friend Nobody Likes (if watching over that person is a chore). See also Women Are Wiser and Parenting the Husband, for when a wife plays this role to her husband.