From Bulbapedia, the community-driven Pokémon encyclopedia.
Boon Island (Japanese: ゆうぎじま Friendship Island) is the second island in the Sevii Islands archipelago. It is east of Knot Island, and west of Kin Island. The water to the east of the island is flowing too roughly, therefore, it cannot be surfed upon.
On the island is a small town, known as Two Island, where, despite its size, there are many activities and important things. The town is a trading post for many foreign goods, and as such has a market stall where items that are unavailable in the Kanto region, including Moomoo Milk and Lava Cookies, are sold.
Boon Island also houses an old woman, who many of the island's residents consider kind of creepy, on Cape Brink, north of the island. If the player's original fully-evolved starter Pokémon has maximum friendship, she will teach them one of the elemental variations of Hyper Beam.
On the west side of the island's small town is a newly-opened Game Corner, owned by Lostelle's father. Over the Wireless Adapter, players of Pokémon FireRed and LeafGreen can play mini-games with each other or with someone who has Pokémon Emerald.
It is also the home of the FireRed and LeafGreen move relearner, who will teach a move learned by a Pokémon at an earlier level that it no longer has or never had, in exchange for two TinyMushrooms or a Big Mushroom. This differs from the other Generation III games, who require Heart Scales, due to the lack of wild Luvdisc in the game.
There is a small inaccessible island directly southeast of the main island that looks identical to the Navel Rock sprite on the town map.
Locations on Boon Island