The Tagalong is usually a child, or at least the youngest member of the squad. They're goofy, irresponsible, and have a selective hearing condition when it comes to commands like "stay here," "don't move," and "go get help."
In some cases (which I don't usually indulge in) the Tagalong is a female reporter. And let me tell you, they're nothing but trouble. It's not common for a Tagalong to the be the main character, but you might find that in stories like Stuff of Legends, where the main character seeks out the hero, rather than being the hero.
The Tagalong is useful as comic relief and sometimes as a catalyst for the hero, motivation to actually do something against the Big Bad or the Evil Corporation, either because something happened in the Tagalong's tragic past or because the Tagalong has been kidnapped, a la Return of the Joker. (Spoiler alert - just about any incarnation of Robin is a Tagalong.)
Above we have:
The Flash from the Justice League cartoon - easily the youngest and least responsible of the team, though not technically a child.
Wilykit and Wilykat from the 2011 reboot of Thundercats - they tag along with the gang even after being told multiple times to bug off.
Tim and Lex Murphy (the kids) from the 1993 classic Jurassic Park - grandchildren of Dr. Hammond (the old guy that started this mess) and victims of Selective Hearing Syndrome.
Elan from the webcomic Order of the Stick - a largely useless member of the team for a long period, who serves almost exclusively as a means to get the rest of them in trouble and move the plot forward.
One of the best things I've found you can do with a Tagalong is to help the protagonist learn something about himself. For example - in Maleficent (an excellent remake, if I'm allowed to have an opinion) the former main character Princess Aurora serves as a secondary character (and a Tagalong in a lot of ways, though she doesn't strictly fill that role). Without her help, the titular protagonist Maleficent wouldn't have discovered more about herself than she'd already known.
How would you use a Tagalong in your story?