Mjolnir, spelt M, J, O, L, N, I, R, is the hammer of the Norse God Thor. Thor is the God of thunder in both Norse Mythology and in the Marvel comics.
There are no restrictions for lifting the mighty hammer other than being very strong in the Norse Mythology, but in the comics, one must be worthy of its power to lift it. Some characters who have lifted Mjolnir except Thor and Odin, Thor's father, are Jane Foster, Beta Ray Bill, Captain America and Vision.
But what makes one worthy enough to lift the mighty hammer?
First, we have to understand why Thor and the other characters I mentioned are worthy. Thor and Beta Ray Bill are willing to make necessary sacrifices to keep the world around them safe. Jane Foster and Captain America have a nobel mind and a strong heart. Squirrel Girl tries to understand people around her and her opponents and tries to solve their problem in a way that nobody is harmed. Vision is an AI, so he can lift it easily as he is not a living being.
But we must keep in mind that Mjolnir has a mind of its own. It even talked to Jane Foster once when she asked why she was worthy. It even declared Thor and Odin unworthy once because of some reason. Beta Ray Bill was also declared unworthy because he was destroying planets to starve Galactus. This is also one of the reasons why Captain America can only lift it during a huge battle.
So, to lift Mjolnir, one must be ready to make the necessary sacrifices to keep the universe safe, should have a Nobel mind and a strong heart and must have good intentions. It also depends on the kind of situation that the person is in to lift it.
