King Leopold I of Belgium (who played an significant role in European
history of his time).
His thoughts were influenced through the British postal system,
which issued its first post stamp already in 1840.
This brought a change in the way to send letters. First the receiver paid
for post but from then the sender paid in advance.
On July 1st. 1849 the first post stamps in Belgium were launched.
Belgian stamp - Source picture: Wikipedia
As you can see, the first stamps had a picture of King Leopold I of Belgium
wearing his uniform. But there were rumours that the King himself didn't
like the design...
More about the look
The stamps didn't carry the name of the country. They only could be used
The value was written in French (not in Dutch).
Nowadays the Belgian Post issues new stamps for special occasions.