When creating a new character in a RPG whether that be one used by a player or as a key non-player character a lot has to be considered.
Extending the storyboard approach I've talked about before you have to look for a hook that either connects the character to the ones used by the players.
If this is a new Player Character then the immediate questions that spring to mind are -
How do they know the PCs?
Why would the existing PCs be happy for them to join the party?
What reason does the new character have for joining the party?
Why is the new PC only now looking to join the party?
Each of these takes a different look at the background not just of the new PC but also of the existing PCs.
Yes a lot of them can be brushed over and in some respects you could just ignore all of them "for the sake of the game" but choosing to skip these questions doesn't help unless the game is simply a series of one-shots. If this is a story driven campaign then it makes sense to ask these questions.
Not only does this give the players a chance to write some of their own history it also allows them to shape the setting and gives the party a reason for being beyond the simple "heroes needed to stop big bad" approach.
In parallel to the party cohesion this can build it also gives the GM a chance to look at elements of the plot that previously may have been overlooked.
Does this new character have a background plot point that connects to the overall plot?
Will the villain be prepared for this change in the party? If not how does that change their plans?
Or maybe the villain killed his father and he now needs to prepare to die...