As we all know Demacia was founded as a refuge from magic after the Rune Wars. Even though Practice of Magic is banned from Demacia now days, yet Garen's family was able to find a lot healers to cure their family member and Lux, Garen's sister practice in Magic as well; Shyvana a half human half dragon is a guard of Jarven IV....ect.
From the Picture Riot Provides we can tell the classic interior and the Golden chain, shows that this champion is indeed locked in Demacia. From my guess This champion could actually be one of the magicians who founded Demacia but locked him/her self away, because he/she knows how dangerous magic can be and how powerful he/she is. But it's also unlikely for riot to release two magicians at same time, so what if this Demacia Founder trained him/her self to be a semi-warrior and mage during the period of time he/she is locked away?
Sorry my English is not very good.