A slightly more formal or emphatic variant of だけど, used to connect contrasting clauses. Common in spoken and written Japanese.
He is young. However, he is very reliable.
This restaurant is expensive. But the taste is reliable.
でも is a casual sentence-initial 'but', while だけども typically joins two clauses within a single sentence.
Derived from the copula だ + けども, an older emphatic form of けど (conjunction).