I think it’s safe to say that tieflings were the breakout hit of 2e AD&D’s Planescape setting (kudos to Wolfgang Baur, apparently), and they continue to be fascinating adversaries, NPCs, and PCs.
As for Pathfinder, there is a whole book out there, Blood of Fiends, to help you bring your tieflings to life—not to mention chapters in the Advanced Race Guide and Pathfinder #25: The Bastards of Erebus as well. So you don’t need my help there—you’ve got everything you need. (Fans of big random tables will especially love BoF and Pathfinder #25 for all the variant abilities and physical features you can roll up with your percentile dice.)
Instead, I’ll move to the larger thought: Tieflings are both signs of and commentary on the otherworldly influences in your world. Their very presence, or lack thereof, is a barometer of how porous the boundaries are between the planes. In a low-magic fantasy campaign, tieflings might be rare to the point of being singular. Even in a more magic-rich campaign, they might not be common—the Dragonlance setting had little truck with fiends, for instance, and the Dark Sun setting was even more isolated. But in cosmopolitan settings open to infernal and abyssal influences—Golarion chief among them—tieflings might be almost commonplace. Certainly they’re commonplace enough in nations like devil-worshipping Cheliax to form their own substrata of society.
In which case, then, adventurers would do well to learn about the various strains of tieflings out there, because prevalent tiefling bloodlines offer clues as to the prevalent evil influences in region. Hellspawn tieflings are a sign of a nation that takes its cues and perhaps even its marching orders from Hell. The demon-spawned pitborn are more likely in areas where witches make dark bargains, and are a sign that law, order, and the protection of religion has broken down in that region. If a party runs into the kyton-spawned shackleborn, they are almost guaranteed to find sadomasochistic secret societies as well; if they find hungerspawn or beastbrood, they should look for oni or rakshasas hiding in plain sight amid luxury and power. When you understand the tiefling, you have an insight into the spiritual vulnerabilities of that tiefling’s family line, his hometown, or even his entire nation.
The city of Salvation prides itself on offering second chances and redemption to all. Good-aligned tiefling flock there, forming a sizeable minority. Most join the golden-sash-wearing Order of Eris, whose many vows of brotherhood and self-restraint keep the all-tiefling membership on the straight and narrow. But the Order takes a dim view of any tiefling who refuses to join their ranks, and their means of persuasion will turn from words to cudgels and worse in short order.
Ifrits are honored in genie-loving Tikrit, but tieflings are not. That’s why Mikael Shem surrounds himself with ifrits—with his scarlet hair he fits right in. His gang of otherworldly footpads has most of the Southgate quarter in their control—an impressive feat in a city that punishes stealing by lopping off a hand. But Mikael lives in fear that his fiendish blood and cloven right foot will be exposed, and he will kill anyone—anyone—to protect that secret.
Maralyn of Avona has a good heart and a wicked eidolon. Though she hides her tufted ears and scale-spotted legs, she can’t hide the bloated, toad-like form her eidolon takes, nor its bloodthirsty nature. Any encounter with Maralyn as an ally will feature her eidolon encouraging the most expedient, morally suspect course of action, no matter what harm it may cause to others. Any encounter with Maralyn as an enemy means facing off against an eidolon determined to carry out her commands in whatever way spills the most blood.
—Pathfinder Bestiary 264
(I had weird formatting issues during the letter K on both my blogging platforms, so forgive me if you have trouble reading the Kangaroo & Thylacine entry or the Kappa entry. Switching browsers may help.)