You are right, so many comments about it's unfair for arjen that he got only half of mana than arien...this is shoujo manga of course it will focus more to girl perspective...if you really want to read a ridiculous power of guy mc then go read shounen manga...there were a lots manga that potrayed that type of guy mc that it's so cliche...while the female mc just act like a damsel of distress waiting for a hero to save her...
Have a different amount of mana doesn't mean that the power is not equal for both twins...I think their power compliment each other since they both have one & same guardian...mana is needed for magic power & maybe it was arien's duty to have healing power & protect arjen with magic power while arjen fighting with a sword. Arjen himself said that he will excel in swordmanship.
like i said i hope he receive something cool or anything else (like being a great swordmanship), maybe saying that is unfair go in the wrong way, but what i really are trying to say was: i don't want that arien be that cliche female "help help I'm in trouble" i like that she have a lot of mana (i think too that se will use to healing and protection) but i want too that they focus in arjen i little more, like showing in one chapter something about him
a hope that the second prince don't suffer or anything like that, like he put a lot of magic in the twin's, I think he will be sick :/
i hope arjen got something cool, he don't have a lot of mana so it's a little unfair with him, the perspective is already with arien so, a hope he receive something (if i wrote something wrong I'm sorry)