Light
pets in 'Terraria' absolutely add their
glow on top of torches — they don't cancel torches out, they just layer light together. I like to think of torches as the steady background lighting and the pet as a moving lamp that follows me through tight caves. In practice that means if you walk into a dark cavern holding a torch and your light pet is out, the tunnel will be noticeably brighter around you than with just one of those sources
alone. The pet's radius helps fill in shadows and makes spotting traps or ore veins easier, especially when the torch placement isn't perfect.
Mechanically, you can expect normal additive behavior: multiple light sources combine, so a torch on a wall plus your companion's light equals more illumination where they overlap. This is one of those little conveniences that makes spelunking less fiddly — I often keep a pet and a torch active while exploring, and it lets me see both the floor and the walls without constantly re-placing torches. If you like, stack in a mining helmet or Shine Potion for even more brightness; the world quickly becomes almost studio-lit, which is great for detailed builds or late-night resource runs.
Personally, I love the aesthetic of a glowing sprite bobbing beside me while I
dig — practical and cute, the best combo. It turns late-game spelunking into a cozy, well-lit adventure, and I rarely go exploring without both at this point.