Professor Birch is the Pokemon Professor of the Hoenn Region, and a dear friend of yours. He is close to you because your brother is dating his daughter, May. However, this is not immediately known by you, as you first meet him very soon after you have lost your memory. You defeat a Zombie which has attacked him, and he takes you to Oldale town, where you use the Pokemon Center to rest.
You then proceed to Route 103, where he is waiting to give you a MysteryEgg, and information about where to go searching after Landon. While you go on this quest, he remains in Oldale, to repair the PC network and gain contact with other survivors around the region.
He gives you the Pokephone, which is essential to maintaining contact with him, so that you stay up to date on information around the region, and are able to respond quickly to dangers that may arise.
You do not see him very often in the ROM, but he does call you quite often, sending you on missions and giving you tips and advice about where to go to advance on your quest.
He appears a final time in the meeting at the Sootapolis Pokecenter the discuss the plan to invade Necropolis.