You Do Need power plug adapter while traveling from Tonga to Saint Kitts and Nevis.

Here is Why You Need An Adapter

The outlet and plug type used in Saint Kitts and Nevis are D and G and in Tonga is I. This is how they look like.

Tonga

In Tonga, outlets of type I are in use.

Type I

Saint Kitts and Nevis

In Saint Kitts and Nevis, outlets of type D and G are in use.

Type D

Type G