Whenever you’ve got two characters who interact with each other a lot, chances are that one is a foil of the other. There are a lot of reasons for this, but the big reason is that it helps to highlight certain character traits by providing contrast. Because the contrast is the important thing, the relationship between the characters can take a variety of possible forms. It may be that one is the hero and the other the sidekick, or (if they’re villains) perhaps one is the Big Bad and the other is the Dragon. If enemy mine or one of the other frenemy tropes is in play, they might be on opposing sides.
Foolish sibling, responsible sibling is what happens when the character foils are siblings. It’s a subtrope of sibling yin-yang that contrasts the character traits of responsibility and recklessness, duty and prodigality, and how the two opposites somehow manage (or not) to live together and stand up for each other in spite of their differences.
It may be used to set up an aesop, usually along the lines of “be like the responsible sibling, not the foolish one,” but that’s not always the case. It might be that the younger sibling is closer to earth and the uptight responsible one needs to learn how to loosen up. Simon from Firefly kind of fits that mold, though he learns to loosen up not from his sister River so much as from the rest of Mal’s crew.
In a lot of stories, it’s not necessarily meant to send a message so much as set up an interesting dynamic between two equally sympathetic characters. In the movie Gettysburg, for example, Lawrence Chamberlain is the commanding officer of the regiment, and thus has to lead the men, follow military protocol, etc, while his annoying younger brother Tom calls him by name and forgets to salute him, runs around chatting it up with union soldiers and rebel prisoners alike, and generally seems a lot more loose and carefree.
Usually, the responsible character is the older sibling, for reasons that should be fairly obvious to anyone who grew up with siblings. As the oldest child in my own family, I can readily sympathize with the dutiful son, since I more or less was one. That’s not always the case, though. In sitcoms where the middle child is the main character, usually it falls on them to thanklessly pick up the slack (yeah, being the middle child pretty much sucks). Bart and Lisa from The Simpsons are a good example of this.
I played with this a little in Desert Stars with some of the minor characters: as the second oldest, Surayya generally tries to do things by the book, whereas Amina tends to be more mischievous and conniving. Michelle and Lars also fall into this trope, with Lars a carefree academy dropout and Michelle a hardworking (though also fun-loving) mechanic on her father’s ship. Between Desert Stars and Bringing Stella Home, though, Lars completely turns around, so that by Heart of the Nebula (not yet published) he’s quite possibly one of the most responsible characters in the book (and I still have yet to give him a viewpoint … hmmm).
In Sons of the Starfarers, I’m playing with this character dynamic a lot. Isaac is the oldest son, who always knew he would leave on his father’s starship to fulfill the Outworld traditions and seek his fortune as a star wanderer. Aaron, on the other hand, kind of got roped into the whole thing unexpectedly (see Star Wanderers: Benefactor) and hasn’t yet matured. The events of the story will no doubt give him a growth arc, but in the meantime, the dynamic between the two of them is a lot of fun to write.
There are a lot of other issues in that relationship to play with too, such as promotion to parent and always someone better, but I’ll save those for another Tuesday.
