Ambient, or summoned at a one-time nether altar.
Herobrine can appear ambiently at Corruption Levels 1 and 2, or be summoned deliberately at a nether altar, netherrack, mossy stone bricks, gold blocks and four redstone torches, lit with fire. Each altar can only be used once; it stays permanently "spent" afterward.
Once summoned, it stands for 5 seconds, sends a single "hello" message, then vanishes with a lightning strike as the world resets to day. Encountering it unlocks the "hello?" advancement.