A Rune Knight is a very special and rare kind of soldier, who is able to use magic by engaging with an Elemental Witch. By taking the Unity Ring of a Witch, a bit of her magical energy flows into the Rune Knight, giving him the affinity for that element, as well as the ability to use it for limited time. It should be noted that no one knows if there can be female Rune Knights or if it is possible to engage with a Wizard, as there were only Elemental Witches and Roland is the only Rune Knight ever mentioned by name. As Masters seem to have similar abilities to a Rune Knight, there may be a connection between the two classes.
Roland is said to be the first Rune Knight for over 4000 years, which incidently is the same time in which Mage Queen Elicia was banned to Arthania. Roland is also a very special case, as he became a Rune Knight after absorbing the Runic Engine. Even after becoming a Master, his Class still says Rune Knight.