Bulgaria has always been close to Russia.
not “always”
Bulgaria was the first big Slavic empire - starting in the 7th century, though the Bulgar ruling class were Bulgars, a TurkiC people.
The Tsardom of Muscowy’s wars with the Ottomans in the 1700s and 1800s led to first the creation of Bulgaria as a de facto independent part of the Ottoman empire and then the Principality merged with Rumelia.
HOWEVER, in the early 1900s Bulgaria and Russia were on opposing sides:
1. Bulgaria threatened to conquer Constantinople — and the Russians sent troops to help the Turks prevent that (as they didn’t want another Slavic power to threaten Muscowy)
2. Bulgaria was on Germany’s side in World War 1 and 2 - against the Tsardom and then against the USSR.
3. Soviet Russia occupied Bulgaria after WWII and kept it under its thumb
So Bulgaria has definitely not been “always close” to Russia