Latios
Latios, part of a legendary duo of Pokemon with Latias, is a Psychic- and Flying-type Pokemon. Its first appearance was in the previous two Pokemon games, "Pokemon Ruby" and "Sapphire." In "Emerald," it is a Level 40 Pokemon, who is strong against Fighting-, Bug- and Poison-type Pokemon. However, Latios is weak against Dark-, Ghost- and Electric-type Pokemon. Latios is also holding a Soul Dew, an item that increases Latios' special attack and special defense when it is holding it. As a legendary Pokemon, Latios cannot be bred with any Pokemon in the Day Care Center to make an egg.
Finding Latios
Latios can only be found once you have defeated the Elite Four and become the Pokemon champion. After the credits roll, you awake in your house in Littleroot town. View the TV downstairs; when the the program concludes, your mother asks you what color the Pokemon on the TV was. If you answer blue, Latios begins appearing in the wild. Latios randomly moves from route to route and can be tracked using your Pokedex. Once found, Latios runs away after the first turn, so use a Pokemon that knows the move "Mean Look," or any similar move, to prevent it from fleeing.
Fighting Latios
If you are not interested in capturing Latios, you can defeat it to get 211 experience points. As Latios is weak against Electric-, Ice-, Dark-, Bug- and Ghost-type Pokemon, any Pokemon of these types -- or one that knows these moves -- will do additional damage to Latios in battle. When Latios' health bar is depleted, it is defeated. Defeating Latios in battle removes it from the game permanently, so you can no longer catch it or make it appear. For this reason, it's best to capture Latios rather than defeat it.
Catching Latios
To catch Latios, avoid moves that it is weak against. These moves may cause more damage than you anticipate, accidentally defeating it in battle. To capture Latios, lower its health bar until it is red, indicating it does not have much health left. Use a status-inducing move, such as "Spore" or "Sleep Powder," to put Latios to sleep or paralyze it. However, avoid status-inducing moves that cause burns or poison, as these do damage over time and may accidentally kill Latios. Use your strongest Pokeball to capture Latios; it may take more than one Pokeball before it is successfully captured. In addition, save your game before fighting Latios; if you accidentally defeat it, you can reload your save and attempt to capture it again.