Unlock special characters
First, complete Suikoden 4, and make sure to collect ALL 108 stars of destiny. After clearing the game and saving, start a new game for Suikoden Tactics. When it asks, load up the Suikoden 4 clear data.
Hero4: Right before the event at the Laboratory on Kooluk continent, go back to the Island Nations and go to Obel. Speak to Lino under "Gossip", then go to the deserted island, go to the coast, and help Hero4 defeat a few monsters. He'll then join (wielding the name you gave him in Suikoden 4).
Snowe: After recruiting Hero4, go to Razril, talk to the little boy under "Gossip", then help Snowe fight furballs in the back alley. He'll join after the battle.