How do I choose whether Latias or Latios roams Emerald?

Right after the League, your mom asks about the color of the Pokemon she saw on the TV news. Answer blue and Latios takes off roaming Hoenn; answer red and Latias does instead. That one reply is the only thing that decides which Eon Pokemon is yours to chase.

What's the trick to actually catching the roaming Lati?

Cornering it is the hard part, since it teleports away on turn one and only sits at level 40. Use the Pokenav to spot which route it's prowling, fly over, and brush the grass with a trapper already leading so it can't bolt before you get a move in.

Emerald Roaming Hoenn

Not a place you can walk to, but the trail of a wanderer. The Lati twin you left behind at Southern Island streaks across Hoenn's routes, slipping away the instant you corner it.

Roaming Hoenn is in Hoenn. In the story it comes after S.S. Tidal.

ROAMING HOENN MAP

Pokemon Encounters

The Eon twin you chose roams Hoenn at level 40 once the story ends, flashing onto routes only to bolt the moment battle starts. Run it down with a fast Pokemon and a full bag of balls.

✦Special encounters2

#380Latias

Legendary Lv 40

#381Latios

Legendary Lv 40

Legendary

Roaming

#380

Latias

DragonPsychic

Level

EV yield

3 Sp. Def

Acquisition

Legendary

Starts on one of 20 possible routes picked at random, then moves between them as you travel

Sealed locationRoaming HoennView on map

One-of-a-kind. You only get a single shot per cartridge — knock it out or run, and it is gone for the run. Save before engaging.

Latias page →

Roaming Hoenn Guide

Right after the League, your mom asks about the color of the Pokemon she saw on the TV news. Answer blue and Latios takes off roaming Hoenn; answer red and Latias does instead. That one reply is the only thing that decides which Eon Pokemon is yours to chase.
Cornering it is the hard part, since it teleports away on turn one and only sits at level 40. Use the Pokenav to spot which route it's prowling, fly over, and brush the grass with a trapper already leading so it can't bolt before you get a move in.