You Do Need power plug adapter while traveling from Saint Lucia to Costa Rica.

Here is Why You Need An Adapter

The outlet and plug type used in Costa Rica are A and B and in Saint Lucia is G. This is how they look like.

Saint Lucia

In Saint Lucia, outlets of type G are in use.

Type G

Costa Rica

In Costa Rica, outlets of type A and B are in use.

Type A

Type B