Ladyslau Yahajla, grand duke of Lithuania. His role in Belarusian history is quite dubious. Continuing the tradition of early Lithuanian princes (Mindouh , Hedymin), who traded their native pagan Lithuania for Orthodox Christian Slavic Duchy, just to return to their native lands as conquerors and enforcers of the foreign Slavic language and Christian culture,  Grand Duke of Lithuania Ladyslau Yahajla traded the Grand Duchy of Lithuania and his Orthodox Christian religion to Polish Corona and Catholic fate to become king of Poland as Ladislaus II (in 1386) when he married Queen Jadwiga. His catholic wife pursued him into opening the campaign of Catholic conversion in Belarus. We can't state that he started a tradition of betrayal in Belarusian nobility, but he certainly was a bright example of switching sides. Oh well, what else can you expect from Yahajla, who came to power by traitorously killing his uncle Kejstut in 1381 and imprisoning his brother Vitaut. Vitaut (who had never betrayed Belarusians and died as the Grand Duke of Lithuania) escaped and ever since they were in bitter argument and rivalry with each other. Yahajla and Vitaut only made peace once to defeat crusaders in the Battle of Grunwald (1410).

Links related to King Yahajla:

Marriage of Yadwiga and Yahajla
Jagiello from Encyclopedia,com
Culture.PL: Grunwald Days
Songs About Litwins from the Eldest Times to the year of 1434  in Belarusian
Die Schlacht bei Grunwald

Other Relevant Pages of the Virtual Guide to Belarus

Belarusian Castles and Knighthood
Historic Belarusian Battles
Belarusian Statehood 
History of Belarus