Ash's Meltan was the leader of the Meltan group and befriended Ash's Rowlet. Since Lusamine's Clefable were taking care of the other ones (that were multiplying), this one stayed with Ash.
It battled against
Faba's Hypno in the top 16 of the Alola League.
Faba thought it was
Ash Ketchum's weakest Pokémon, so it got Hypno to use Psychic to cheat and select its Poké Ball for Ash before the match started but Ash was fine with the selection. Meltan ended up eating
Faba's Hypno's coin to stop its Hypnosis and finished it off with a Flash Cannon to give it the match victory.
Ash's Melmetal is the evolved form of
Ash's Meltan. It evolved on the docks of the Manalo Stadium after spinning its nut most of the day to call in the nearby Meltan.