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That'd be The Friend Nobody Likes
Most Definitely Not Accompanying Us for the one who ends up joining the adventure despite no one wanting them to.
See also Loser Friend Puzzles Outsiders, where the circle of friends like Bob but others don't get why
This is an inherent part of The Friend Nobody Likes: The trope is the situation as a whole, including the reason the person is in the friend group despite no one liking them.
That's why I point out Loser Friend Puzzles Outsiders as a compare/contrast that sometimes overlaps.
- Just The Friend Nobody Likes: The group like Adam enough to put up with his Annoying Younger Sibling and Millstone Bob.
- Just Loser Friend Puzzles Outsiders: The whole group accepts Bob, even if people don't get why.
- The two overlapping: The group complains about Bob all the time and he's the group Butt-Monkey, but when a bully makes Bob cry there's a Hypocritical Heartwarming moment as they put the bully through a Humiliation Conga.

Often, though not necessarily, this friend will have undesirable qualities.
'to ease typing this, Adam refuses to go without Bob.
the friend nobody likes - Bob is this.
the page has some reasons why either Adam or the group overall keeps Bob around.
some other possibly relevant pages I found while looking.
cloudcuckoolander's minder - if Adam is tasked with looking after Bob, and thus Bob must come along.
Adam is often obligated. "I have to babysit my little brother Bob" for example.
arranged friendship - Adam is often causing an arranged friendship between Bob and the rest of the group,
cool kid and loser friendship - If the group is popular and doesn't want to include Bob for status reasons.
tagalong kid - an often unwanted or unhelpful character who often manages to 'tag along' with the main group.
The Load - a character who's strictly a detriment to the group.
the champion - if Adam is always kind, looking out for, and taking care of Bob, Adam might be Bob's champion'.
Some help. A name for it could be 'package deal', though I am unsure if there is another.