Latias appears when you talk to the Copycat Girl and get her doll from the guy in the Pokemon Fan Club in Vermilion City. Upon exiting the building, Steven will tell you Latias is loose in Kanto.
I'm pretty sure that Latios is the only one of those two who appear in SoulSilver, if you want Latias you gotta trade with someone who caught it on HeartGold.
Yes, Mewtwo, Latios and Latias will be in HeartGold and SoulSilver.
goldenrod
Latias is a Dragon and Psychic type pokemon.
only latias is available
You need to defeat the champion again to make latias reappear.
In pokemon Heartgold
You can't get Latios in HeartGold. You can get Latias, to get her, talk to the copycat in Saffron and find her lost doll in Vermillion city at the fan club and Steven will be out there when you come out. He will talk about Latias for a minute and Latias will be in Kanto just walking around like Raikou.
You can't get to the Hoenn region in HeartGold or SoulSilver. However, Latias can be caught in HeartGold, which is a legendary from Hoenn.
you need your national poke decks to do this look on the map on your pokegear it wil show where latias is
latias or latios will show up on your pokegear map just like raikou and entei
it roams kanto after you get it on the marking map