Menkar is the Cetus 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.
His importance in the story is insignificant.
Menkar was the one of the 3 protector of the Thunder Ruin's guardians... Souma and Yuna decide to fight him to distract him while Kouga, Ryuho Haruto and move on. Menkar is defeated, but survives.
After the war ends, Menkar goes to fight against Pallas, as a hero that know who is the true Athena. As a good guy, he goes to fight in Pallasbelda, but Miller defeats him and destroys his cloth.