Identical to the Shield Breaker used by Roy's partner Marth, Roy charges his blade up for a bit before releasing with a flaring slash (which Marth can't use). The difference between Shield Breaker and this attack is that Flare Blade can instantly KO any foe when fully charged. Roy will take 10% damage when a fully charged attack is released. An excelent edge-guard.
Up + B
Blazer
A rising slash that sends Roy upward a good distance. An opponent hit just when Roy jumps will be sent forward in a blaze of flames (hence Blazer).
Down + B
Counter
Roy protects himself with his sword for a short while. If he is attacked in this time, he will block the attack and counter attack.
Forward + B
Double-Edge Dance
By pressing B three more times, Roy will launch a four hit combo. The attacks can be modified by holding up or down while pressing B, allowing for many different attack combinations.
It is worth noting that, when smashing with a melee item (i.e. beam sword), Marth and Roy swing their sword first, followed by an item swing. Against low-percentage enemies, the first strike is a great check and provides excellent positioning.