No, you are not required to use the same tier characters as your opponent. However, it is highly recommended that you do so in order to have a fair and balanced match. If both players choose lower-tiered characters, the battle may be too easy or too difficult for either player. Conversely, if both players choose higher-tiered characters, the battle may be too evenly matched. If you and your opponent do not use the same tier characters, the game will automatically adjust their levels to be as close as possible to each other. This ensures that both players are using characters of similar power levels, making for a more fair match.