Wenceslaus I (c. 907-935) was Duke of Bohemia from 921-935.
His mother Drahomira was a pagan who persecuted Christians until Wenceslaus took control of the government and exiled her about 924/5.
Wenceslaus I was assassinated by nobles favoring his younger brother Duke Boleslav I in September 935 and almost immediately became popularly believed to be a martyr and a saint.
After Bohemia became a kingdom permanently in 1198 he began to be described as a king, an anachronism, including in the popular song "Good King Wenceslaus".
The song "Good King Wenceslaus" is based on stories that assumed that a good Christian saint must be very charitable to the poor and unfortunate. Thus a preacher in the 12th century (1101-1200) is quoted:
But his deeds I think you know better than I could tell you; for, as is read in his Passion, no one doubts that, rising every night from his noble bed, with bare feet and only one chamberlain, he went around to God’s churches and gave alms generously to widows, orphans, those in prison and afflicted by every difficulty, so much so that he was considered, not a prince, but the father of all the wretched.
And if his earliest biographies were written soon enough after his death with information from people who had known him, they might be more or less accurate about his charity.
But you need an expert on his biography to be sure about what is known and what is legend.