Landon is your character's older brother, and May's boyfriend, in Pokemon Snakewood. He is the Pokemon League Champion and is based of the playable male character in Pokemon Ruby, Sapphire, and Emerald. He saved the region three years before Snakewood takes place, banishing Groudon and Kyogre, and has gone missing after he went searching for the source of the Zombie Plague.
One of the main goals of your character in Snakewood in to locate where Landon has gone to, and rescue him and May from any possible dangers that they have encountered that made it impossible for them to return to you and Professor Birch.
You first hear of Landon having headed for Rustboro city after you arrive in Petalburg. After making it to Rustboro, it is said he went through the Ventendurf Tunnel, and you go through this tunnel after banishing demons.
Once you complete this, he was seen around Mauville city, and Gleis tells your character that you will find more clues once you arrive in New Mauville. However, none arise.
The final place that Landon has been spotted before his capture was Famine's Desert, where he was captured after battling with Famine's monstrous Aerodactyl.
He is mentioned once you leave Fort Draco to return to Mossdeep city, as his signature Pokemon, Swampert, was seen battling the Zombies along with the Inquisition Forces.
After meeting with the council, Meteor flies you out to Necropolis where Landon, May, and Wally are captured, frozen in carbonite. In the ROM, you are only able to save Landon and May. Wally is left behind. Landon and May exit the building, and are not seen for a long while.
Landon appears in a confrontation against Barqesh in the postgame as well. This is the last you see him in the ROM.