Ok first off lets pretend its the overlords turn he moves adjacent to the Knight Class and he Plays Guard and defeats the monster. Is it allowed even if on the overlords turn to use Advance if fatigue points are still available? My group thinks it should not be allowed because its the overlords turn but nothing on the card states otherwise.

At first glance i'd say YES. Let's try to elaborate a bit.

GUARD stipulate: "When a monster moves into a space adjacent to you, exhaust this card to interrupt that monster's activation and perform an attack with a Melee weapon. After this attack is resolved, if the monster was not defeated, it may continue it's activation".

ADVANCE state: "After defeating a monster with an attack using a Melee weapon, exhaust this card to move up to your speed and perform an additional attack".

So, when the monster entered a space adjacent to the Knight, the Knight exhausted his GUARD card to perform the attack. Then let's say he killed the monster...IMO that would activate the ADVANCE trigger and could exhaust this skill card to move and attack again.

I think that it is fitting in the Knights role, as a tank, to try and block monsters from hurting his comrades.

Thanks I thought I was interpretting this correctly.

Sounds right. It is not really overpowered if you think abut it. The Knight needs to not use Advance during his/her own turn and the overlord should be aware of the Guard and Advance being unused before he/she starts sending a wounded or low HP monster near the Knight.

Yeah and it'll cost the Knight fatigue so the OL is getting closer to apply some additional hurt on the reckless Knight!