You Do Need power plug adapter while traveling from Kiribati to Bulgaria.

Here is Why You Need An Adapter

The outlet and plug type used in Bulgaria are C and F and in Kiribati is I. This is how they look like.

Kiribati

In Kiribati, outlets of type I are in use.

Type I

Bulgaria

In Bulgaria, outlets of type C and F are in use.

Type C

Type F