McDohl, the Hero from Suikoden, appears in the game if you have collected all 108 characters from the first Suikoden. Load your previous Suikoden game data when prompted after starting a game. Play through the game until you have gone over to Gregminster and have met their leader. Then, go to Banner Village and talk to Ko, the boy who is dressed similarly to you. After, go over to the east side of Banner where the dock is located. There will be a man, Gremio, blocking your path (if not him, it will be Ellie). Return to Ko, and he will help you get past the man. That should lead to McDohl. McDohl and Viktor, like others, will make their second appearance in the series.

If you load data from the first Suikoden without having all 108 characters, he will be called RcDohl. But other than the name he is the same person. McDohl (or RcDohl), after returning Ko to banner village, will only join you again if you talk to him in Gregminster. If you have him leave your party afterwards, you have to go all the way back to Gregminster to ask him fight with you again.

To get McDohl in your party, you do not need all 108 characters from the first game. However, when you get him due to a text parsing error in the game, the first letter in McDohl will be the first letter of his name in the first game (for instance, my character was named Daros, so in the second game, his name is DcDohl). This has nothing to do with the number of characters.