I imagine the warlock as a real nobody initially—no magic powers, no real prospects, stuck in some dead-end village and resigned to being a minor NPC for the rest of their life. Maybe they’re frustrated because their childhood best friend has moved on to bigger and better things and they feel abandoned and unimportant.
Perhaps they end up taking a job as a maid at the local tavern in the dim hope that they’ll get a few lines in the next time a party passes through. Unfortunately, it’s not the sort of tavern that seems to attract adventurers—it’s too well-lit, too kitsch, and popular mainly with old people who flock to it for Fantasy Bridge Night and Bingo twice a week.
One day, however, a retired adventurer drops by to compete in the annual Fantasy Bridge competition finals (is that a thing? I don’t actually know what Bridge is) and our NPC cleans his room. As a tip, the adventurer reaches into his treasure chest and tosses them a little handful of tacky jewelry and a couple gemstones—explaining that it’s too much trouble to have it all appraised or exchanged for actual currency. Our NPC grumbles silently, but accepts the tip.
As they leave work that evening, they take off their uniform and change into their street clothes, only to realize that they don’t have any pockets. Maybe the treasure clinks or rattles inside their lunchbox and they’re afraid of getting mugged, so they slip some into their shoes, another piece into their bra or the waist of their loincloth, and then slip the necklace on over their head.
Just as the pendant settles over their heart—the metal is warm, why is it so warm?—they get knocked flat on their ass by the sudden manifestation of their New Best Friend, an immensely powerful and socially inept force that is utterly enthusiastic to finally, finally have a BFF, but has little to no understanding of what that actually entails (but is more than happy to improvise).
I imagine the patron being chaotic neutral and very annoying—a little bit Q from Star Trek (with a sprinkling of Luxwanna Troi), a little bit Venom, a little bit Robin Williams Genie, a little bit Om from Small Gods. Altogether too meddling in the warlock’s personal life, emotionally volatile, and a little jealous. The warlock is embarrassed and reluctant to be associated with them, but the pact is for life—the amulet didn’t say “best friends for awhile”, after all. It said “forever”.