Mirfak is the Perseus saint of the Century XXI. He is loyal to Athena, but thinks she is Aria, and Mars is an Athena new ally, so, obey his orders.
Mirfak is sent to protect the Water ruins in Rosan. How Ryuho lives close of the ruins, he goes to attack the guardian of it (Mirfak) to do the Kouga, Yuna and Haruto mission more easy. But, Ryuho doesn't beat Mirfak, so, Yuna, Kouga and Haruto arrive there to defeat him. But, Mirfak uses his shield to turn the 3 into stone statues; if Ryuho could defeat Mirfak before this, the 3 would still be human (Ryuho is most weak than the 3, so, his weakness caused it). So, Ryuho uses his power to defeat Mirfak and breaks the Medusa Shield.
After the war ends, Mirfak goes to fight against Pallas, as a hero that know who is the true Athena. Maybe, Kiki fixed his shield to be used again.