Namor would be blockable.
Generally character count abilities fall into two categories
1. The more the merrier. If it is something that gives a bonus based on multiple things in play, then "character" implies uniqueness and it ignores duplicates. In future releases, a card like Teamwork would be worded as follows:
Each of your fielded characters gains +1A and +1D for each other active character you control that shares a team affiliation with it.
The word active lets you know it happens no matter how many copies of that die are in the field.
2. Leave me alone. If it is something that requires it to be the only thing in the field, then "character" is typically actually referring to individual dice and it will consider other dice, even if they are duplicates, to be "present" for purposes of determining whether the effect will trigger. This occurs on cards like the solo attack Wolverines, in the future cards like this will typically use language similar to, "If a Wolverine die attacks alone it cannot be blocked."
Namor falls into the More the Merrier category.
-The Dice Masters Rules Team