The big grayish building in the north-central part of town is the Nacrene City Museum/Gym. As you try to enter, N will walk out, and he will wish to battle you.
PKMN Trainer N
Pokemon:
Pidove (Lv. 13) Normal/Flying
Tympole (Lv. 13) Water
Timburr (Lv. 13) Fighting
Type Weaknesses:
Pidove are weak to Rock, Ice, and Electric. They are immune to Ground and Ghost moves. Nothing exceptional here; your Fire/Water simian/starter will function better, the Grass-type here.
Tympole is weak to Grass and Electric. This one likes to confuse you. To heal this, just switch the Pokémon for instant healing. If possible, use your Grass type, but DON'T use your Fire type.
Timburr is weak to Flying and Psychic. This one is somewhat more powerful compared to most Lv. 13's but isn't special otherwise.
When you defeat N, he will leave.